In top of which, installing a heat pump, due to the way they're being used, would require replacing all of the existing central heating pipework with larger diameter pipes and all of the existing rads with much bulkier ones, in order to compensate for the much cooler temperatures that the system will operate at.
What is always glossed over, in this push for heat pumps, is that they can't be a drop-in replacement for a hot water boiler; using them as such is a very poor use of the technology. Why are we proposing to use hot air to warm up water, which then heats up the air again, when we could just use the air directly? But then, that would require the admission that a "heat pump" is just a crippled HVAC system, which would then require them to explain why HVAC is bad for the environment until you're using it to make tepid water, at which point it's suddenly good.