How exactly would a heatpump be incompatible with existing pipework and radiators? I see them installed into all kinds of houses, summer cottages and whatever no problem.
Meanwhile the efficiency (COP) of a heat pump drops the higher its temperature differential is. So trying to keep the water loop temperature the same is a non-starter. (The same problem can happen with high efficiency condensing natural gas boilers which have much lower flue temperatures)
(From an American perspective although I'll convert the units) - older radiator systems were designed for water temps of around 80C, room temp around 20C, so that is 60C (60 K/Kelvin, really) delta-T. Whereas if a heat pump wants to put out 50C water for half the delta-T (30K), you then need twice the amount of radiator surface area to transfer the same amount of heat. The system will work fine in the shoulder seasons when not much heat is required, but at the coldest nights design temperature it will not be able to keep up and your indoor air temperature will then drop despite the heat being on.
(Pipe sizing enters into it as well but isn't as straightforward to explain)
(And re-arranged the furniture a bit + stopped using the radiator as storage...)
That said, most properties in the UK don't have this problem. They _do_ likely need bigger radiators which comes with a cost, but a lot of these houses would benefit from just upgrading from their single panel rad with no fins.
I'm not sure any of that makes heatpumps technically impossible, but it makes them more expensive and less efficient
I guess it's cheaper to not have to install air ducting or indoor units if you can reuse existing pipes & radiators. But if you need to replace the radiators anyway, the cost might not be that much different? You'd get more precise temperature control, and cooling in the summer.
Mini-splits are noisy and have major problems getting heat distributed over a whole house.
In our case we couldn't install ducting downstairs (no space between the floors), but it was simple and cheap to do so upstairs because there was attic / roof cavity space in which to run the ducting.
A big advantage of the mini-split is that it gives you precise temperature control in each room, where as our ducted system only has a single central thermostat, and on/off control in each room. This is problematic for the bedrooms, which tend to overheat at night in the winter when bedroom doors are closed, while it remains cooler in the hallway where the thermostat is. I'm currently looking into an upgraded control system ("Airtouch 5") which should solve this by putting wireless thermostats in each room.