Sweaty new-builds with no air con

We're slowly getting there. MVHC imo is going to be the next big one that goes in the regulations.
The concept is, you seal the house up so that airflow doesn't happen unless you want it to.
Then set up heat exchangers with ducting between the rooms.
So fresh air can be brought in, cooled/heated by the stale air going out.
And it also lends itself well to aircon systems.

There is also Part O of the building regs, which uses calculated shade on south-facing windows to minimise solar gain in the hottest months.

Eh this place I'm in is from the 30's and despite the 'renovations' to turn it into a disturbing number of bedsits for the size of the building, it still has it's original 'historically significant' wood framed, single pane sash windows, so along with the complete lack of insulation and piss poor ventilation, it ends up damp and freezing cold in winter too.

There are so many older buildings that have been done up 'slum lord' style like this in the uk, it's actually kinda scary how common they are. A lot of temporary/emergency housing is in buildings like this and the landlords make a huge amount of money by shoddily subdividing and renting to the council, and save an equally huge amount by avoiding doing vital repair and maintenance work till court ordered.

Your ancient glorified cave sounds wonderful, ngl.