Gridded potential evapotranspiration calculated from United Kingdom Climate Projections 2018 (UKCP18) regional climate model (RCM) ensemble at 12 km resolution over the United Kingdom for the years 1980-2080 under the Representative Concentration Pathway 8.5 (RCP8.5) scenario. This dataset contains two potential evapotranspiration variables: daily total potential evapotranspiration (PET; kg m-2 d-1) and daily total potential evapotranspiration with interception correction (PETI; kg m-2 d-1). PET and PETI were calculated for each member of the UKCP18 RCM perturbed parameter ensemble. The units kg m-2 d-1 are equivalent to mm d-1. The data are provided in gridded netCDF files. There is one file for each variable, for each ensemble member, for each decade.