Optimising city-scale data to improve care for 10.6 million people

OneLondon was established to improve health and care outcomes across the capital, serving a population of 10.6 million people across five Integrated Care Systems, 35 NHS trusts and around 1,385 GP practices.

London’s health and care data sits across many systems and standards. Deep, clinically rich data exists in primary care, acute, community and mental health settings, supported by multiple electronic patient record platforms. Bringing this data together safely and consistently at population scale had long been a challenge.

Without a single, secure and standards-based data service, it was difficult to plan services, reduce waiting lists, support population health management or enable research across London. Fragmentation limited confidence in insight and made it harder to turn data into action at system level.

OneLondon needed a core data service that could ingest primary care data at scale, harmonise it into a common model, and publish trusted, analysis-ready datasets back to Integrated Care Systems and into the London Secure Data Environment for research.

Answer Digital was commissioned to design and build the London Data Service as the core data engine for OneLondon.

We worked closely with OneLondon, Integrated Care Systems and delivery partners to design a service that could operate at city scale while meeting strict requirements for security, governance and data quality. The service ingests general practice data from both EMIS and TPP sources, harmonises it into a common data model and publishes it back to each Integrated Care System through secure, controlled access.

As confidence in delivery grew, our role expanded beyond engineering into programme planning, service readiness and roadmap support. We helped shape the transition from delivery into business-as-usual operations, ensuring the service could be adopted, operated and extended safely as additional datasets are onboarded.

The London Data Service is now operating at significant scale.
As of late September 2025, more than 75% of GP practices across London had signed up, with primary care data flowing first and community and mental health datasets next on the roadmap. Each Integrated Care System receives consistent, well-coded data products to support analytics, planning and improvement.

At a London-wide level, the service feeds the London Secure Data Environment, enabling approved research on de-identified, standardised datasets spanning primary, acute, community and mental health care as onboarding progresses.

The programme has reduced reliance on proprietary platforms by establishing a London-owned data architecture, creating a clearer path to cost control and innovation. Alongside delivery, we are supporting service readiness activities, including documentation, onboarding and the rationalisation of legacy business intelligence services into a coherent London Data Service model.