Backend and API development in London
A London payments or wealth platform rarely needs a new backend. It needs the one it has to be dependable: a ledger that reconciles, webhooks that post once when a provider retries, and a clear answer to what happens when the database fails over in the middle of a batch. Most of the codebases we see were written to reach the market first, which was the right call at the time, and now carry idempotency gaps that only show up at quarter end.
We start by writing down what the system actually does against what your team believes it does, then fix the handful of paths that touch money first. Every change comes with the test that proves it, and the architecture notes are written as we go, so the next engineer can follow the reasoning.
Financial services drive more of London's software demand than any other sector. Payments companies, challenger banks, wealth platforms and brokers need ledgers that reconcile, APIs that stay correct when a partner retries, and apps customers trust with their money. Most of them reached the market fast, which was the right call at the time, and now run systems built for launch rather than for the size they have become. The work is making those systems dependable without stopping the business that runs on them.
Insurance is its own case. The specialty insurers, managing agents and brokers around Lloyd's still run much of their work on ageing policy administration platforms and spreadsheets that travel between firms by email. Lloyd's has moved away from Blueprint Two, the market-wide programme that was meant to digitise placement and claims, so each firm is modernising its own systems one integration at a time. The City's law firms and consultancies sit alongside them, holding decades of documents and few ways to search them by meaning.
Media, retail and the public sector bring different problems. Publishers run subscriptions, paywalls and content systems that have to hold up on a busy news day. Direct-to-consumer brands outgrow their first shop as they start selling into Europe and the US. Suppliers to councils and NHS bodies build services that have to work for every resident, including people using a keyboard or a screen reader. Hiring is the common constraint. ITJobsWatch puts the median advertised salary for a software engineer in London at £98,750 over the six months to September 2026, and banks, fintechs and the large technology firms compete for the same people.
Working with us from London is straightforward. Four hours of every working day overlap with yours, set across your morning, so stand-up, code review and the decisions that follow happen live. You talk to the engineer who writes the code, in your own Slack, GitHub and Linear. The rest of the day moves forward in pull requests you can read at your desk, with runbooks and architecture notes written as the work goes.
We are the right fit when you have a defined piece of work and need senior engineers on it this month rather than next quarter. Start with a fixed-price two-week piece and judge us on what ships. We add capacity while a London hire is still in progress, and we work white-label under your name for agencies.
Four ways this arrives.
We read the code, write down what it does versus what everyone believes it does, and give you the list of the five things most likely to page someone at night. Then we fix those first.
Usually queries, not architecture. We profile under real traffic, fix the plans and the indexes, and only then discuss whether anything needs splitting apart.
A mobile client, a partner integration, a public API. We design the contract first, version it properly, and write the docs your consumers will read.
Jobs that silently vanish, retries that duplicate charges, a queue nobody monitors. We make it idempotent, observable, and boring.
A moderated messaging backend where sends acknowledge in-band and the review runs behind them.
Read the write-up →Asked by London teams.
How do you work with teams in London?+
Four hours of every working day overlap with yours, set across your London morning, so stand-up and code review happen live. We join your calls and work in your tools: Slack, GitHub and Linear. You talk directly to the engineer who writes the code. Outside the overlap, the work carries on in pull requests you can read at your desk, and runbooks and architecture notes are written as we go.
Do you build for London fintechs and insurers?+
Yes. For payments companies, challenger banks and wealth platforms we build ledgers that reconcile, webhook handling that stays correct when a provider retries, and React Native apps with biometric sign-in and step-up checks for risky payments. For insurers, managing agents and brokers around Lloyd's we replace spreadsheet workflows with internal tools: bordereaux intake, referral queues and approvals, connected to the policy systems you already run.
How does your rate compare to hiring in London?+
ITJobsWatch puts the median advertised salary for a software engineer in London at roughly £99,000. Our published rate is $35/hour, with a $5,000 minimum engagement. There is no recruiting time and no employment overhead, and we can start within days of the first call. Most clients begin with a fixed-price two-week piece at $2,800, so you judge us on working code before committing to more.
Can you work white-label for a London agency?+
Yes. London agencies bring us in when a client project needs more senior backend, mobile or infrastructure work than the team has free. We work under your name, in your repositories and your client's tools, and you stay the face of the project. The NDA comes before specifics, the code belongs to the client from the first commit, and runbooks and architecture notes are written as we go.
Can you work in our existing codebase?+
Yes, and it is most of what we do. We do not require a rewrite as a condition of working with you. If we think a rewrite is genuinely the right call we will say so, with the reasoning and the cost, and you can decide.
What if the backend is in a language you do not list?+
Then we will tell you. We are useful in Node, Python and TypeScript. We can read Go and PHP well enough to migrate off them. We would not take a Rust or Elixir project and learn it on your budget.
Do you write tests?+
For anything with money, permissions or data loss in it, yes. We do not chase a coverage number. We will tell you which parts are covered and which are not in the handover notes.
Who owns the code?+
You do, from the first commit. Repositories live in your organisation. If we set them up, we transfer them before the first invoice.
What happens when the engagement ends?+
You get runbooks, architecture notes and a recorded walkthrough. We stay available for questions for a month at no cost, because a handover that needs us on retainer is not a handover.