Modern housing in the UK is ugly as hell. The only upside is it is so poorly constructed it won't be around for that long.
1970s-80s: stone cladding.
1980s-90s: cream brickwork, porticos
1990s-00s: boxes with white upvc windows, red tiles, garages
2020s: grey plague - grey windows, grey doors, grey plastic faux-wood cladding, grey roof tiles, car 'ports'.
The current awful trend would be hilarious if it wasn't actually dangerous. And by that I mean modern UK houses are very, very well insulated. And we recorded our first 40C temperature in England. There's a reason why houses in the Mediterranean are absolutely not grey, and are very definitely white. There have even been one or two TV programmes where they've measured these houses and apartments having indoor temperatures of high 20s low 30s C, long after the sun has set.
If your home is meaningfully affected by the outdoor temperature it is very much not "well insulated".
Windows are a bit special in that you can get solar gain through them, but modern windows can have coatings that filter UV and IR from entering a building.
People and electronics are heat sources inside a home, but pretty minimal ones. Any reasonably speced ventilation system a modern home needs to exchange outside air will easily handle keeping a modern house cool.
In common language usage, UK houses are insulated to stop heat escaping in our mostly temperate climate for energy efficiency reasons, not to stop heat entering.
Everyone opens their doors and windows in the UK allowing the heat in. Aircon is rare, but increasing. So houses stay hot long after the afternoon peak.
https://www.bbc.co.uk/news/uk-62126463
https://www.bbc.co.uk/news/uk-politics-23180965
and so on.
I think you need to use a more precise term, maybe something like heat retention.