FfS , when we go on holiday we have aircon, pools and beaches.
How on earth would lying by a poolside or sitting on a beach make you less likely to get heatstroke?
I do think there is a bit of hysteria in this thread, possibly because COVID has broken people's brains a bit. Japan where I live regularly gets temperatures hotter than this in the summer, and I don't usually use aircon in my flat while working during the day. Japanese schools also mostly didn't have aircon until fairly recently.
Switch schedule around as much as possible (ie, get your outdoor time in the later part of the day if you can). When outside, seek shade and carry a big dark umbrella with you. Sip water, obviously. Just sensible habits. I've never had heat stroke. Heat stroke, IME, is usually what happens when people are sunbathing or hanging out on boats; you can choose not to do those things on a very hot day.
The main concern I would have about the UK is that some wiring and road surfaces etc. may not have been created with such high temperatures in mind. I'd avoid doing significant travelling, and back up my documents regularly etc if working on a PC.