Abstract High temperatures and heatwaves are associated with significant impacts on human health. With continued global temperature increases, extreme thresholds relevant to health will be exceeded more frequently. This study provides an updated spatial analysis of heat-related mortality for the UK, using the UK Climate Projections (UKCP18) at 1.5 °C–4 °C global warming levels, and embedding population and demographic data from the recently released UK Shared Socioeconomic Pathways (UK-SSPs). Climate change will lead to an increase in heat-related mortality in the future, exacerbated by increased exposure due to increasing population. We find an increase from ∼1400 average annual deaths in the near-past (1990–2019) (95% CI: 1299–1485), to ∼2500 (2304–2794), ∼3700 (3280–4214), ∼8200 (7376–9072) and >18 000 (16 690–20 394) average annual deaths at 1.5 °C, 2 °C, 3 °C and 4 °C respectively (assuming no adaptation). This is considered a high-end estimate due to the assumption of high population growth (UK-SSP5). Older populations are shown to be most vulnerable. A large proportion of heat-related deaths (76% (74%–79%) with 1.5 °C global warming) are attributed to more moderate (1 °C–5 °C) increases above regional temperature thresholds as opposed to extremes. Our results provide a timely update that can serve as a first step to supporting future UK climate policy and risk assessments. Future research considering nonlinearity in the health response to heat exposure is vital.