Simply because all DHW is currently from an electrically heated cylinder, ewwwwwwwww.
I know zilch (or rather less) about CH/DHW. Any experience/suggestions on a good unit?
3 sinks, most probably one at a time. Shower (often). Bath (rarely). Washing machine. Dishwasher.
CH radiators at a later date.
Possible cost of install (cavity wall)? Gas/hot/cold pipes close to hand.
Cheers.