It’s a distinct type of humidity — and the streets in England are very narrow. Everything is densely packed — at least in a city like London. The buildings, many of which are quite old, are not designed for hot weather, but instead amplify it.
The old buildings were designed for hot weather - built before fans, much less air conditioning, and with windows that open.
Of course, the rich would decamp to their summer residences, but the poor always had to sweat.