[ubercow13]

It's unclear if this list is exhaustive but from https://www.bbc.co.uk/news/science-environment-57988023

>Heathrow Airport was the first place to break the 40C mark, hitting 40.2C at 12:50 BST but several other places also passed 40C during the afternoon, including Gringley on the Hill in Nottinghamshire and St James's Park, Kew Gardens and Northolt - all in London.

[ibejoeb]

I think that's the wrong article. It says "we are likely to see temperatures of 40C" but doesn't claim that anywhere actually did.

[notahacker]

https://www.bbc.co.uk/news/uk-62217282