It's not necessarily the heat pump, but the internal remodelling to fit larger rads/underfloor heating, and better insulation.

Fundamentally, after that you're using an energy source 3x as expensive as gas, so... any time the COP is less than 3 you're losing money. Oh, guess when you need the most heating? That's right, when it's cold. I don't give a flying what the COP at 10 C is, because waste heat from everyday activities keeps the house warm at that. I care about the COP at -2. On a becalmed day when the output of circa. 24 GW of wind capacity is < 1 GW. This absolutely happens.