> There is no need to 'rip it out and replace the whole system' to use a heat pump; an air to air heat pump as used in many Scandinavian homes can be fitted at much lower cost without disturbing the existing central heating at all.

I understood that heat pumps work more efficiently in homes that are a) well insulated, and b) where the heating system is designed to work at lower temperatures (bigger radiators/underfloor heating and appropriate pipes). Whether or not a & b are must-haves or nice to have I'm not 100% sure about - I guess it depends.

(see e.g. https://energysavingtrust.org.uk/advice/in-depth-guide-to-he... - Designing and operating your heat pump system)