You didn’t answer the question Is that a thing people do in the UK? I’ve never heard of anybody doing that. I’m not sure why you would do it.

Unlikely to be normal, but I have been doing it at a couple of places I park at this year, since I'm there for a while, have to park in the sun and the places are semi-enclosed (ie, it is fenced so random people not walking around but not secure compound level). Also, my car is 17yrs old, I got it for free and don't keep valuable items inside. I prefer leaving the window open to coming back to 38degC inside. No aircon.