I was quoted £15000 max for an installation and could claim back on govt grant, but no official figure set as not yet available in my area of UK. Not really an incentive.

The main problem is that the temperatures aren't the same as gas boilers deliver. Typically 82*c flow (out water temp)
and 71*c ( back water temp) return water temperature to the boiler. Hence the present radiator sizes required to heat the space ( room volume). Heat pumps deliver about 65*c flow which will mean up sizing radiators by at least 50%.
Ie a single will be a double and a double will be a treble. That's if they are sized correctly of course. Later for the basic calculations.
The outdoor units are much quieter than the older ones but take up a lot of space and still need power to run. An average of 8kw at the low end. That's 32 amps and many fuse boards are at the limit now so compared to a boiler at max 5 amps there are problems with the 'load' already.
The calculations for radiator sizing which I use are.
Living and dining rooms 5 BTU per cubic foot.
Bedrooms 4 BTU per cubic foot.
Bathrooms and ensuites 3 BTU per cubic foot.
BTU = British thermal units
3412 BTU = 1 kw or 1000 Watts.
Ie a lounge 10ft long x 15ft wide x7ft tall.
10 x 15 x 7 = 1050 x 5 = 5250 BTU or thereabouts
Different calcs for different rooms but same process.
If your heating with a boiler needs to be on longer than 20 minutes to warm your drum your rads are undersized I'm afraid.
I oversized mine by 20 % so 5 minutes and my place is steaming.
Most merchants brochures have radiator sizes and outputs so check if your rads are the correct size for a boiler system.
A rough measurement and type of radiator ( K1,K2 etc) will be close enough.
Then add 50% to the correct size for a boiler system to a heat pump system and watch your wall disappear for when you get a heat pump.
That's aside of an electric element in your hot water cylinder at
12amps (3 kw) to trim your hot water to 55*c and the problems gets bigger. Sadly hmg have not done any presearch or spoken to the pros and the tree huggers don't even get it either I'm afraid.
Pm me if you have any questions ref your rads.