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. Full details about this dataset can be found at https://doi.org/10.5285/eb5d9dc4-13bb-44c7-9bf8-c5980fcf52a4