Estimating Undocumented Migration in the UK Using an Exclusionary Health Care Policy Reform

Estimating the number of undocumented migrants poses significant challenges, particularly due to data scarcity. This briefing outlines an approach based on 2014 policy reforms in England's National Health Service (NHS), which restrict healthcare access for migrants lacking valid residency documentation—commonly referred to as the NHS "hostile environment". These reforms primarily affected access to primary care, such as General Practitioner (GP) services. We present our methodology, the data used, and the insights it could offer if expanded with more granular data. Preliminary findings suggest that the reforms led to a slight reduction in new GP registrations in practices serving large migrant populations, implying that new arrivals were less likely to register. This trend, when paired with arrival data, could help estimate undocumented migration flows through a multiplier method.