I used to say (once acclimatized) under 40C, sensible clothing, water, going slowly, and staying indoors 11-4 things were OK. But we had AC everywhere.
Over 40C, and just being was tough.
Without AC - which I would guess much of Europe doesn't have everywhere - I don't fancy those temperatures. It will be very tough. We used to cool the house to 27C.
The one thing that Brits on holiday could take note of is covering skin helps keep you cool. I used to wear linen blend trousers and a loose shirt. Much cooler than shorts and vest top.