UK Tmax versus Sunshine are well correlated. Cleaner Air. More Sunshine.

This graph is Tmax % of normal versus Sunshine% of normal. The curved lines are the Loess curves from [R]. Data retrieved from here.

Why would sunshine be higher? Cleaner air in the UK (until China and India started to burn a lot more coal) thanks to the Clean Air Acts of 1956 and 1968.

The loess curves bottom out around 1968/1972.

UK Tmax vs Sunshine (Annual)_Tmax_Sunshine