The humid weather in the UK is definitely more difficult to tolerate than, say, the weather in Greece, Malta or Spain, which is much drier.
That said, I climbed up through a traditional village in Greece a few years ago and we ended up pouring water over ourselves rather than drinking it, because we were so hot that we actually felt quite ill.

It's just this year I seem to be more effected by it than ever before, and quite noticeably so, and the only thing that has changed is meds and weight.

I will drink more but I can't promise to stay out of the sun