Here in the UK, we don’t experience extremes like you do. On average, you might get temps of -5 degrees c (23) in the depths of winter and about 30 degrees c (86) in summer. (Cooler in Scotland)
The issue here is three-fold.
1. It’s damp. Really damp, so summer can get humid and sticky and horrible and winter can feel bitterly cold like the air is gnawing into your bones.
2. It rains a lot, or is overcast all the time, so that dampness only increases and lack of general sunshine (particularly from about October to about march) adds to a perceived coldness.
3. British people and the infrastructure aren’t used to extremes of weather and tend to complain/collapse as appropriate.
Take houses. A modern house will have central heating as standard, but that’s from radiators. It’s not blown air or climate control. It won’t have air-con. Very few houses in this nation had air-conditioning as standard.
The thing is, a lot of our houses are old. Normally built around 1900 or so, or sometimes older, so the existing structure has often been adapted. Conveniently, I live in a fairly standard 1900s terrace. It’s not got cavity wall insulation, it’s solid brick. It can get sticky hot in summer thanks to direct sunlight on the slate roof turning the house into an oven, and freezing cold in winter because of the inefficient heating system and the drafts that tend to sneak in despite your best efforts to keep them out. Even with double glazing and oodles of insulation in the attic, it can still be chilly.
Since a lot of flats, particularly in London, are normally older buildings converted, they’ll be chilly in the winter, with inadequately sized radiators and drafts, and hot in the summer, often due to light on the slate roofs and not enough insulation to keep the heat out.
Also, every fic author ever loves a chance to make their otp either sweat a lot and remove clothes or shiver and snuggle under a blanket XD
Hope that answers your question a bit!