The traditional argument for deferring core replacement rested on risk avoidance. If the system works, do not touch it. For OSB Group, operating six distinct lending brands each with its own product configurations, pricing structures, and customer journeys, that logic held until the market stopped cooperating.

When interest rates began moving sharply, the pressure became impossible to ignore. Brokers expect products to be withdrawn and repriced within hours of a base rate change. Borrowers expect decisions quickly. But on a monolithic system where every change, however small, triggered a formal project and weeks of interlinked testing, OSB was measuring its response time in days. In a market where a competitor moving faster could capture the broker relationship and the customer that came with it, that gap was not an operational inconvenience. It was revenue walking out of the door.

The industry record on large-scale core banking programmes is sobering. In the UK, the consequences of getting it wrong have played out publicly. TSB's 2018 core migration resulted in approximately £200 million in direct costs, a £48.65 million regulatory fine, and 5.2 million customers affected, some locked out of their accounts for months. It remains the most visible reminder in the UK market of what is at stake when a big bang cutover goes wrong.

For OSB Group, that outcome was never a risk worth taking. As Chris Healing, Lead Product Owner in Mortgages at OSB Group, describes it, a core replacement is not simply a matter of installing new technology. It forces an institution to unpick every piece of financial logic that has been embedded in its systems for decades and rebuild it from first principles, with zero tolerance for error.

Consider what that means in practice for a specialist lender. How is an interest rate calculated across a live book of thousands of borrowers? What happens to a loan schedule when a customer takes a payment holiday, switches to interest only, or reaches the end of a fixed term and rolls onto a new product? How does the bank handle a base rate change, not as a theoretical exercise, but in real time, across every active account simultaneously? Chris Healing puts it plainly: there are around a hundred distinct processes that a core replacement forces you to rethink from first principles. Each one feeds directly into regulatory reporting, treasury positions, and hedging calculations. Getting any one of them wrong is not a recoverable error.

The technical challenge is only part of the picture. Finance teams need to co-design the financial logic from scratch. Regulatory and compliance functions need to validate every process that moves. Implementation partners need to understand not just the technology but the specific market the institution operates in. The speed of a programme like this is ultimately determined by the quality of the people involved and the ability of the technology to deploy what those people are designing. Without both, timelines extend, costs compound, and the institution carries the full weight of mid-migration exposure with neither the old system nor the new one fully operational.

OSB understood this. It is precisely why they did not approach modernisation as a single programme to be delivered in one go. Instead they chose to build from scratch on a composable foundation, migrating capabilities incrementally and maintaining control at every stage. In the video below, Chris Healing sets out in his own words why that decision was not just pragmatic, but essential.

Composable banking mitigates modernisation risk by changing the shape of the programme entirely. Rather than building on a monolithic foundation where every component is interdependent, a composable core allows an institution to configure, test, and deploy individual capabilities independently.

That applies whether an institution is migrating from an existing system one product line at a time, or building an entirely new platform from scratch with the same modular discipline. In either case, the risk is distributed across time rather than concentrated in a single go-live moment. A problem in one area cannot cascade across the entire institution. The programme moves at the pace of the institution's confidence rather than the pace of a monolithic release cycle.

OSB Group chose the more ambitious path. Rather than purely migrating capabilities from their existing system, they built an entirely new platform from scratch, rethinking every experience they wanted to create for customers and colleagues rather than replicating what already existed.

But the composable principle held throughout. Savings products went live first, then Buy-to-Let through Rely, then residential mortgages through Precise, each capability configured, tested, and deployed independently before the next move.

As Chris Healing describes it, that approach opened up something a legacy-constrained migration simply could not: a core that was genuinely visible rather than a black box, with API and data flows that were cleaner and testing environments that were easier to navigate. For a programme rebuilding around a hundred distinct financial processes from first principles, that clarity was not incidental. It was foundational.

Mambu's composable core banking platform made that ambition achievable in practice. Because the platform is built to adapt to the institution rather than constrain it, OSB could define their own product logic, interest calculations, and account structures from first principles without being bound by what a legacy system would allow. And because Mambu's open, API-first foundation connects cleanly with the surrounding ecosystem, OSB could integrate the origination, decisioning, and servicing tools they needed without the interlinked dependencies that made every change so costly on their old stack. That is what turned a greenfield ambition into a deliverable programme.

Composable banking provides the foundation. But even the best airport needs reliable aircraft to get a full lending operation off the ground. nCino sits at the lending layer of this architecture, bringing intelligent workflow orchestration and automated decisioning across the full lending lifecycle. For OSB's underwriters, the manual intervention and siloed handoffs that defined the legacy process give way to a single configurable environment where decisions are faster, more consistent, and fully auditable. Credit risk, property risk, and fraud assessment feed into a unified workflow rather than sitting in separate systems requiring manual reconciliation at every stage.

Composable banking provides the foundation. nCino provides the intelligence. But the most compelling argument for this approach is not theoretical. It is already live across the UK lending market, in institutions of different sizes, different starting points, and different ambitions, each proving that composable modernisation delivers on its promise.

OSB Group's transformation is the most comprehensive example currently live in the UK market, but it is not an isolated one. Across the lending sector, institutions with different histories, different scales, and different starting points are demonstrating that composable modernisation is not a single institution's experiment. It is becoming the established path for UK lenders serious about competing in a market that will not slow down to accommodate legacy infrastructure.

Leeds Building Society is the UK's fifth-largest building society, managing over £30 billion in member assets. After more than two decades on a legacy core, the Society launched a pilot digital savings product on Mambu's cloud-native platform in under 12 months, the first milestone in a multi-year modernisation that will extend to its mortgage offering. For an institution of this scale and heritage, that speed of delivery reframes what incumbent lenders can realistically achieve on a composable platform.

Read the full Leeds Building Society story →

Interbridge Mortgages launched in early 2024 with Mambu as its foundation from day one. Within nine months it was live with lending products. Within 15 months it had introduced 18 mortgage product variants, surpassed £300 million in lending, and helped over 9,000 customers, all while maintaining a 4.9 Trustpilot rating. In a specialist second-charge market where decisioning speed and product breadth are the primary competitive differentiators, composable banking was not just the technology choice. It was the commercial strategy.

Read the full Interbridge Mortgages story →

Interbridge Mortgages launched in early 2024 with Mambu as its foundation from day one. Within nine months it was live with lending products. Within 15 months it had introduced 18 mortgage product variants, surpassed £300 million in lending, and helped over 9,000 customers, all while maintaining a 4.9 Trustpilot rating. In a specialist second-charge market where decisioning speed and product breadth are the primary competitive differentiators, composable banking was not just the technology choice. It was the commercial strategy.

The outcomes OSB Group has achieved did not happen because one platform solved every problem. They happened because the right capabilities were brought together deliberately, with each party in the relationship doing what it does best and none of them trying to do what the others do better.

That is the model worth understanding. Mambu provides the composable core, the stable, cloud-native foundation that adapts to the institution rather than constraining it. The intelligent lending layer, delivered by nCino, sits above it, bringing the workflow orchestration and automated decisioning that turns that foundation into a front-to-back lending operation. And the implementation expertise that bridges platform capability and institutional reality, understanding not just the technology but the specific regulatory environment, financial logic, and market dynamics that UK specialist lenders operate within, is what makes the whole thing deployable at the speed the institution needs rather than the technology alone would allow.

What that combination makes possible is something that procurement processes rarely account for: the ability to move at pace on genuinely complex, highly regulated lending products without sacrificing the precision, auditability, and control that the market demands. Mambu's open platform makes nCino's integration clean and fast. nCino's intelligent decisioning makes Mambu's composable core immediately productive for lending operations. And the implementation layer makes both deployable at the pace the institution needs. The result is not a technology stack. It is a delivery ecosystem, and that distinction matters.

For OSB Group, that model translated into something that Samina Khan, who leads the data and enterprise function at OSB, describes in terms that no vendor briefing could replicate: taking a transformation from a PowerPoint exercise into the reality of a scale platform in the market, acting with pace and intent, and delivering on ambition. That is not a technology outcome. It is a partnership outcome, and it is the distinction that matters most for any UK lender weighing up this decision.