FastAPI Development Services for UK SaaS, AI, and Data Teams
FastAPI development is the building of high-performance, async Python APIs and microservices with the FastAPI framework, the service layer modern SaaS, data, and AI products run on. MicroPyramid has 12+ years of Python and API work and ships these for UK startups and SMBs: async APIs, microservices, and ML/LLM serving layers built for real concurrency.
Why UK Teams Choose Us for FastAPI
Three reasons UK founders and product teams pick MicroPyramid over a local agency or a generic offshore team
Privacy-Aware APIs
API surface, logging, data retention, and third-party calls are designed with privacy and auditability in mind from the first sprint. AWS eu-west-2 (London) available for UK data residency.
GBP Billing via Stripe or GoCardless
Invoices in GBP, collected via Stripe or GoCardless. No currency conversion headaches, no wire-transfer friction. Billing that fits how UK businesses actually operate.
Clear Ownership, Not a Ticket Mill
Our engineers directly own every engagement. You will not be passed to a delivery layer of junior developers. The engineer who scopes your API in discovery is the one who builds it.
UK APIs and Integrations We Build With FastAPI
A UK product is rarely just its own API. It sits on LLM providers, Stripe, HMRC, NHS services, Companies House, and the accounting platforms UK businesses already run. These are the integrations we build most often, and what goes into doing them properly.
LLM and AI Provider APIs
OpenAI, Anthropic, AWS Bedrock, self-hosted models
UK teams adding AI features rarely want to bet the product on one model vendor. We put OpenAI, Anthropic, AWS Bedrock, or a self-hosted model behind a single FastAPI gateway that streams answers over server-sent events, retries and fails over between providers, and records token spend per customer. Prompts and responses are logged with personal data redacted first, so a data subject request never means trawling raw transcripts.
- Server-sent event streaming to web and mobile clients
- Retries and fallback across model providers
- Token budgets and cost logging per tenant
- Prompt and response logs with personal data redacted
HMRC Making Tax Digital
HMRC Developer Hub APIs
Accounting, tax, and landlord software submits records to HMRC through the Making Tax Digital APIs. We build the HMRC integration in FastAPI: OAuth 2.0 against the HMRC Developer Hub, the fraud prevention headers HMRC expects on every call, sandbox test scenarios, and the quarterly-update workflow.
- HMRC OAuth 2.0 user-restricted token handling
- Fraud prevention header collection and validation
- Obligations, quarterly updates, and final declaration flows
- Audit trail and safe retries for every submission
NHS and Health Interoperability
HL7 FHIR UK Core, NHS login, NHS England APIs
UK healthtech products integrate with NHS England services such as the Personal Demographics Service FHIR API and NHS login, and with GP and hospital systems through FHIR UK Core profiles. Pydantic models map cleanly onto FHIR resources. We build the integration services and the technical evidence NHS onboarding expects, alongside your clinical safety officer.
- FHIR R4 resource validation against UK Core profiles
- NHS login (OpenID Connect) integration
- Access audit logging for patient records
- Technical evidence for NHS onboarding
Companies House Business Verification
Company data for B2B onboarding
The Companies House public data API and streaming API give you company profiles, officers, and persons with significant control. We build a FastAPI onboarding service around them, so B2B customers are checked against the register once, their records stay current as new filings land, and every lookup is logged for your support and operations teams.
- Company, officer, and PSC lookups with caching
- Change monitoring via the streaming API
- Onboarding forms pre-filled from register data
- A lookup log behind every business account
Stripe and Payment Webhooks
Stripe or GoCardless, subscriptions and one-off payments in GBP
Subscription and marketplace products in the UK usually take payments through Stripe, sometimes with GoCardless alongside it. We build the payments service in FastAPI: webhooks verified before anything changes state, idempotency keys on every charge, a subscription model that copes with out-of-order events, refunds, and a nightly reconciliation job your finance team can trust.
- Webhook signature verification and replay protection
- Idempotency keys on charges and refunds
- Subscription state that survives out-of-order events
- Reconciliation job against provider payouts
Accounting and Payroll Platforms
Xero, Sage, QuickBooks, FreeAgent
UK SMB software rarely lives alone. We build FastAPI sync services against Xero, Sage, QuickBooks, and FreeAgent APIs with OAuth token rotation, rate-limit-aware queues, and conflict handling, so invoices, contacts, and payments stay consistent across systems.
- OAuth token storage and rotation
- Rate-limit-aware background sync
- Conflict detection and resolution rules
- Per-tenant sync status your support team can see
Our FastAPI Development Services
Six services covering the full lifecycle of high-performance Python APIs, from async builds and microservices to ML serving and migration
High-Performance Async APIs
Async FastAPI services for UK SaaS, proptech, and AI products where load arrives in bursts: month-end reporting runs, campaign-driven sign-up spikes, and LLM calls that must not hold up other requests while a slow third-party API responds. Python keeps your data and AI tooling in one language.
- Async/await concurrency
- Pydantic v2 validation
- OpenAPI docs your partners can consume
Microservices Architecture
Carve a UK platform out of a monolith one bounded context at a time: billing, onboarding, notifications, reporting. Each service gets its own contract, pipeline, and audit trail, so it is always clear which APIs each part of the product depends on.
- Strangler-pattern extraction
- Docker and Kubernetes
- REST plus queues (SQS, RabbitMQ, Kafka)
AI & ML / LLM Serving APIs
Streaming LLM endpoints, RAG services, and model-serving APIs with the controls UK buyers ask about: prompt and output logs you can redact, a documented record of what personal data reaches a model provider, and a human-review route for decisions that matter.
- Model and LLM serving
- Server-sent event streaming
- Redaction and audit logging
Migration to FastAPI
Move legacy Flask, Django REST, or synchronous APIs onto async FastAPI endpoint by endpoint, behind the same URLs your UK partners and mobile apps already call. Contract tests prove each cut-over before traffic moves.
- Phased, low-downtime migration
- Contract tests against the old API
- Performance audit first
Maintenance & Support Retainers
Security patching, dependency upgrades, provider and API-version churn (LLM model retirements, HMRC API deprecations), and feature work for FastAPI services in production.
- Security and dependency updates
- Provider version upgrades
- Feature work alongside your team
Cloud Deployment & DevOps
Deploy FastAPI to AWS eu-west-2 (London) when UK data residency matters, with CI/CD, structured logs, tracing, and autoscaling. Infrastructure as code, so your auditors and future engineers can see exactly what runs where.
- AWS eu-west-2 (London) deployment
- CI/CD with GitHub Actions
- Infrastructure as code
FastAPI, Django REST, or Flask? How to Choose
The three frameworks solve different problems, and real UK products often mix them. Here's the honest version, so you pick the right tool instead of the trendy one. We help you make the call in discovery, before any budget is committed.
Choose FastAPI when…
- You need a high-performance, type-safe API or microservice
- You're serving AI/LLM features, RAG endpoints, or heavy async I/O
- OpenAPI-first contracts and Pydantic validation are priorities
- You want a lightweight service, not a full web framework
Choose Django REST when…
- You want a full web app (admin, auth, and ORM) plus its API
- You're building a SaaS, portal, or content-driven product
- Convention-over-configuration and a large ecosystem matter most
- Raw async throughput is not the dominant constraint
Choose Flask when…
- You have a small, simple service with minimal dependencies
- You want maximum freedom over structure and libraries
- Async and built-in validation are not requirements
- You are maintaining an existing Flask codebase, not starting fresh
Need a full Django web app rather than a standalone API? See Django Development for UK Teams, or broader Python work: Python Development Services.
Best Fit For
- UK SaaS, proptech, and AI product teams that need high-throughput, auditable Python APIs
- products integrating LLM providers, HMRC MTD, NHS FHIR services, or Companies House data
- teams serving ML models or LLM features that need streaming, logging, and human review
- migrations from slow Flask or Django REST APIs to modern async FastAPI
Not the Right Fit When
- simple CRUD admin tools where a full async API offers no real benefit
- engagements looking only for staff augmentation without delivery ownership
- pure frontend or marketing-site work with no API or backend complexity
- teams wanting an API rewrite with no measurable performance or scaling problem to solve
Need broader Python work beyond APIs? See Python Development Services, or serving AI features inside your product, see AI Feature Development.
Our FastAPI & Python Stack
Modern async technologies and proven patterns, deployed to AWS eu-west-2 (London) for UK data residency
Core Framework
Data & Storage
DevOps & Cloud
How to Get Started as a UK Team
We recommend starting with an API Discovery Sprint: clarify the workload and architecture before committing to a full build. All engagements billed in GBP.
API Discovery Sprint
Recommended start
Clarify the workload, data contracts, throughput targets, and architecture before committing to a full build. Billed in GBP, delivered with daily async updates and a fixed estimate at the end.
- Workload and throughput analysis
- API and schema design
- Architecture and scaling plan
- Migration assessment if relevant
API Build Sprint
Ship a production-ready FastAPI service or set of microservices with async foundations that are genuinely ready to scale, not a throwaway prototype.
- Production-ready async API
- Pydantic models and OpenAPI docs
- Clear deliverables and timeline
Maintenance & Retainer
Ongoing FastAPI maintenance, performance tuning, and feature work end-to-end.
- Security and dependency upkeep
- Performance and scaling work
- Retainer or time-and-material
Selected Work
FastAPI services and API products we have built and shipped, each linked to its case study.
DiscoveredBy
AI search visibility platform that tracks how brands appear in ChatGPT, Gemini, Perplexity, Claude, and Grok, built with FastAPI, SvelteKit, and AI agents.
Read case studyOnlineCompiler.io
Sandboxed code execution platform for 12 languages with a REST API, WebSocket support, an embeddable widget, and MCP support for AI agents.
Read case studyIntentport
Our voice and text AI agent for websites: answers from live business data, books appointments, and sends qualified leads to the CRM in five languages.
Read case study
CREDITABLE
Employee financial wellness platform covering savings, workplace loans, and repayment workflows, built on Django and FastAPI.
Read case studyFastAPI Development FAQs for UK Teams
Straight answers to what UK founders and CTOs ask us before starting a FastAPI project.
What drives the cost of a FastAPI project?
The main drivers are scope and endpoint count, the complexity of your data model, how many external integrations are involved (LLM providers, HMRC, NHS services, payment and accounting platforms), whether you need real-time or streaming features, and the compliance evidence your sector expects. A single microservice is a very different job from a multi-service platform with several external integrations. We give a fixed estimate in GBP after a short discovery sprint rather than running an open-ended hourly meter.
Can you build HMRC Making Tax Digital integrations?
Yes. We build Making Tax Digital integrations in FastAPI: OAuth 2.0 against the HMRC Developer Hub, the fraud prevention headers HMRC expects, sandbox test scenarios, obligations and quarterly updates, and an audit trail behind every submission. HMRC runs its own checks before granting production credentials, and we prepare that evidence with you.
Do you build NHS-integrated or FHIR-based healthtech APIs?
Yes, on the engineering side. We build FHIR R4 services in FastAPI against UK Core profiles, integrate NHS login and NHS England APIs such as the Personal Demographics Service, and produce the technical evidence NHS onboarding asks for. We are not a clinical safety authority: your clinical safety officer signs off the safety case, and NHS England runs onboarding.
Is FastAPI fast enough compared to Node.js or Go?
For most API workloads, yes. FastAPI runs on async Starlette and Uvicorn and delivers throughput in the same class as Node.js and close to Go for typical I/O-bound APIs, while keeping Python’s ecosystem for data and AI. Go still wins for extreme CPU-bound or ultra-low-latency cases, but for AI service layers, data-heavy APIs, and most SaaS backends, FastAPI is fast enough and far quicker to build.
Why is FastAPI the standard for AI and LLM backends?
FastAPI is async by design, so it handles the long-lived, concurrent connections that LLM calls and streaming responses require, and it sits in Python, the native language of AI tooling like LangChain, vector databases, and model SDKs. That makes it the natural place to build RAG endpoints, model-serving APIs, streaming chat, and agent backends without a separate stack.
Can you migrate our Flask or Django REST API to FastAPI?
Yes. We audit the existing API, identify async and performance bottlenecks, and migrate to FastAPI incrementally, running services side by side and cutting over endpoints without downtime, rather than risking a big-bang rewrite that destabilises the product your UK customers rely on. Many engagements start as a migration and performance audit sprint.
Where will our API and data be hosted?
Wherever your customers need it. For UK products we usually deploy to AWS eu-west-2 (London), multi-AZ, so personal data, backups, and logs stay in the UK. We document every third-party processor each endpoint calls, including LLM providers, because that is where UK residency promises usually break. Data protection is considered from the first architecture session, not bolted on before launch.
How does billing work for UK teams?
We invoice in GBP and collect via Stripe or GoCardless, with engagements structured around fixed-scope sprints or a monthly retainer.
Do we own the source code and IP?
Yes. You own all source code and intellectual property we produce for you. Everything is committed to your repositories as we build, so there is no lock-in and no handover risk if you later bring the work in-house or move to another team.
Ready to Build a Fast, Reliable API?
If you need a team to build or scale FastAPI services (with GBP billing and AWS London hosting), we are ready to talk. No junior handoffs, no ticket mills. Just high-performance API engineering for UK teams.