Gfs18 again has this weird gap between 850s and 2m temps. But not in France where 42C shown.
It seems to be an error with this modelling. Looking at the cloudcover charts it shows a lot of clear skies with 20c+ 850s.
The thing is, whilst it's underdoing 2m temps, it's also underdoing the dew point, which is in turn means it's underdoing CAPE. Hot, moist flow over the UK means high CAPE values and thunderstorms. None of that is modelled, even when more moisture is added from the advancing trough.
Hard to have much faith in the general output that it is churning out at present given the obvious discrepancies.
Crewe, Cheshire
55 metres above sea level