Having 8mm pipework (microbore, which was fairly commonly used when central heating was being retrofitted) connected to standard sized radiators with a flow temp of 35 degrees is unlikely to actually heat a building.

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.

Older houses might need higher temperatures, but insulated ones might be fine with 40-50. My house is slightly over 20 years, and most of the time, we are completely fine with the radiators just lukewarm. If it's like -10 outside, they need to get warmer.

Many old-style radiators only work with high water temperatures. Uninsulated pipework can "waste" a lot of water temperature before you even reach the radiator

I'm not sure any of that makes heatpumps technically impossible, but it makes them more expensive and less efficient

What the other comment said, but more technically, the amount of heat transferred is proportional to delta-T. So older radiators have less surface area, relying on hotter water to transfer enough heat energy to make up for the heat loss of the building.

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)

I can't be the first small-brained Canadian that stuck a fan in front of those euro apartment bloc radiators on a cold day.

(And re-arranged the furniture a bit + stopped using the radiator as storage...)

We're about to get a heatpump (in the UK) and our pipework is fine but we need to replace pretty much all the radiators, both the old ones, about 45 years old, and the newer ones, about 15 years old, in order to be able to heat the house to a sensible temperature.

Is there reason why retaining radiators is preferable to an air heat pump (either ducted air or mini-split)?

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.

The roof cavity is not large enough for hidden ducting and the ceilings in older houses low enough that exposed ducting would be a head impact hazard. So you'd end up needing one installation per room. Expensive, especially when the expertise isn't there, and mostly used by commercial users.

Most old houses are not ducted and retrofiting them is very expensive, capital repair.

Mini-splits are noisy and have major problems getting heat distributed over a whole house.

We actually have both, retrofitted about 5 years ago into a 1980s house. A Toshiba mini-split downstairs and a Carrier ducted system upstairs. The indoor units of our Toshiba mini-split are actually very quiet: barely audible unless you really crank up the fan speed.

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.