I haven't read all this yet, so may have been said. But ignoring air conditioning, etc, it's also acclimatisation. I saw a pet thread some time ago, suggesting that dogs also live in hot countries. But they are acclimatised to it. We send horse competing abroad, at least 6 weeks early, in air-conditioned stables at first, to get used to more heat. People are the same- they need weeks to build more red blood cells and sweat more easily. I think physiological differences get ignored and the UK weather can change very quickly, without a gradual build up.