Yes by heat pump I mean one of those fridge size things. They include hot water tanks. I have one of those and it’s around €4-5000 eur so within the grant money from a scrapped oil burner. Mine produces both 50deg water for radiators and 35deg for underfloor via an electronic shunt (about €500 extra) because I have underfloor heating on one floor and radiators on the upper floor.

It also connected directly to 4 pipes hot/cold tap and hot/cold radiator water making an easy swap.

Are you saying the space requirement of 60x60x180cm would really be the problem?