Are the boilers typically connected to water-radiators?.. I assume so based on the word "boiler".
There are heatpumps that are used to heat water so it would be a slot in replacement..
The pump is a drop in replacement unless you have 8mm "microbore" piping, at which point the lower temperature times restricted flow rate becomes a problem in terms of getting enough heat through.
And a dirty tank of water in the attic to act as a "in-house water tower" because only one tap may be connected directly to the mains. Really archaic.
Would you say modern (i.e. <10 years) are properly insulated and built or are there still... shortcuts?
and that house still has the sewage stacks on the outside of the house, as do almost all homes in Bath and environs.
You don't see them on new builds, I think, probably because the pipe going from inside to outside would reduce insulation effectiveness.
Otherwise people try to retrofit narrow drain pipes in the walls which are prone to clogging or give you poor flushing performance. Or outside big enough pipes outside interior walls where you get to hear every flush/shower unless you build a box around that. Easier to just run it outside if you can configure your bathrooms that way.