The British have a slightly different definition of "hot" than Americans. 25C is a heat wave. (Although recent hot summers are revising this.)
When an American speaks slightingly of the British definition, they are often told "Oh, but Americans have air conditioning everywhere and we don't".
This is sort of true. All shops have it, cars have it, many houses have it (almost all in parts of the South), even the New York subway has it. But it's not the complaints when it's 33C that get us (or at least me); it's the complaints at 27, which is a temperature most of us think of as ideal, perfect for opening the windows to catch a breeze.
But there's one other flaw. Even though everything is air conditioned now, many of us didn't grow up with it. My parents have always had it at home, but until I was a teenager, I never got to experience it in the daytime. School had no air conditioning (new ones do, but it's too expensive to retrofit). June and September were spent fanning ourselves with paper and feeling sweat slowly drip down our temples.
July and August were, for me, spent at camp. No, not a nice cool camp in the mountains (except for one summer). I spent my summers at the local day camp. This is an exercise in child torture and was even more so in the less enlightened 1980s, when no one thought of smearing kids with sunscreen (unless they were at the beach) and the sport water bottle was yet to be invented. It was 8 hours of standing in blistering hot sun, playing sports, with the occasional period devoted to arts and crafts and 2 blissful periods per day in the pool. The point of this sadistic activity was to exhaust the children so completely that they'd go home, possibly manage some dinner, then sleep for 12 hours, thereby letting parents forget that their children had a two-and-a-half month summer vacation.
There was no concept of "too hot". Possibly if it broke the century mark, they might relax a bit, but 90F was considered perfect weather for a game of softball. New York's weather worked against us. The usual summer pattern is hazy, hot sunshine, occasionally broken by a gigantic late-afternoon thunderstorm. There are relatively few cloudy days, and hardly any truly rainy ones. I can be sure of this, because if we were so lucky to have one, regular activities got cancelled and we got to go bowling or roller skating. That happened about 2-3 times per summer. I have photos of myself where my hair is partly bleached blonde from the sun--I was outside that much.
Kids today don't get it quite so tough (spoiled brats, I tell you, I bet they get decent food at lunchtime too). But 20 years ago they thought all this was good for us. (And that Kool-Aid and Popsicles were good snack foods.) Frankly, all it did was reinforce my utter hatred of sport, but it did toughen me to the heat.