If your home is meaningfully affected by the outdoor temperature it is very much not "well insulated".
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.