[Jochim]

It might work theoretically but that doesn't mean it should be done in many cases. Where I live electricity is nearly 3x the price of gas. While some heat pumps can achieve this in certain conditions it eliminates most of the affordable options which raises the question of how long are you willing to wait before a £10k-50k+ investment pays itself off?

They typically operate at lower output temperatures than existing systems. For example, swapping out a gas boiler for a heat pump will often require replacing all the radiators with much larger ones as well.

IIRC they're also less responsive. You might find yourself keeping the house at 18-20c all day rather than turning off rooms you aren't using. The effect is that you use more energy than you would have otherwise.

[MonkeyMalarky]

Right, it is decision that depends more on the economics of where you live and your usage than the age or construction of your house. Where I live, gas is not an option and there are subsidies to change over to electric and off of heating oil.