I just did some quick calculations (which might be wrong, but maybe not by orders of magnitude) based on reported numbers on the internet. It seems that powering a medium sized air source heat pump purely on solar in a UK winter would require a £34K investment in solar panels, covering nearly 300 square metres. Not exactly viable per property in city-density housing.

City dense will never work since skyscrapers have ridiculously low roof amounts compared to the populace.

Also, as far as heating goes, passive houses should need almost nothing for the winter (still need to heat water for showering, etc. though).

E: Can you share your numbers? A 10kWp plant that I had in mind is ~50m^2 - 30x330kWp panels at about 1.6m^2 each, so did you mean you actually need a 60kWp plant?

Sorry, I didn't save the numbers.. But yeah, sounds right - based on reports that we can only expect 1/5th of the output per panel in winter.

"Passive houses" are not going to happen in the UK for the vast majority of the population in my lifetime!

45N is basically northern Mediterranean; most of Europe does not have similar climate. It does not work fine even at 48N during november-february.