Once again for the non-science people. Solar radiation on the Earth surface has nothing to do with air temperature. It’s to do with atmosphere and how much it passes through.
The sun is higher in the sky in the UK at this time of year so it passes through less atmosphere. You can burn on what feels like a January because to the Earth it’s still June.
Air temperature and sunburn are correlated to the time of year but not causative one to the other.