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.

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

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.

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)