Could you elaborate a bit? My parents live in a not-very-well insulated wooden house from around 1900, and use a heat pump as their primary means of heating the house. This is Scandinavia, so it might be that this house (despite not even coming close to modern insulation standards) still has better insulation than most British houses, but it would surprise me (older British houses are generally built in brick, which should provide a better base level of insulation than a wooden house, and I'd think Britain isn't warm enough that nobody would build a house without any insulation whatsoever).
Maybe I don't understand what you mean by the word "feasible" – they don't have a goal of getting their living room above 23 C at most in winter, and I guess heat pumps are insufficient in such a house if you desire ambient temperatures above that. However, while other means of heating could plausibly bring the temperatures higher, that would end up being very expensive also because of the poor insulation – it's just harder in general to heat a drafty house and keep the temperature up, and I don't see how heat pumps are a uniquely bad choice for homes like that.
Edit: This is coastal Norway, so the climate in winter is quite similar to somewhere like Edinburgh, with temperatures usually above 0 C in January. The heat pumps would probably be insufficient somewhere the temperatures regularly reach -10 or -20 C, but that's a very infrequent event both here and in the UK.
British houses generally have terrible levels of insulation and if built before roughly 1930 are likely solid walled, so have no wall cavity to insulate. There is also the issue that a lot of people live in terraces and semis and there may simply be no suitable place to install a heat pump. When I looked into it for my house a couple of years ago the fitters basically said they couldn't do it because of lack of space.
There isn't that much to elaborate on. Commercial heat pumps just aren't good for the average UK house. They would be "alright" if the costs weren't prohibitive.
I don't know the specifics of your parents. A "wooden house" with a heat pump acting as the primary heating system in a country like Norway sounds fairly bad on the surface. But I don't know the insulation specifics, nor do I know what other heating element might come at play when the heating pump fails to keep up with the heat loss. Also, what heating pump are we talking about?
Heat pumps work extremely well in the Nordic countries and are therefore very popular. That's because due to the cold winters houses are already well insulated, though very old houses less so. Which means the houses are in general reasonably energy efficient to start with, so a heat pump providing 3.5-6kW of heat is generally sufficient. That would be to the main area of the house, so yes the heat pump is the primary heating system (for those who use them) in the sense that it's the heater used for the main parts of the house - think living room, kitchen area. There may well be normal electric heaters in other rooms, if deemed necessary, but other rooms (food storage, bedrooms etc) don't usually need much, if any, heating).
Of course in the Nordic countries you also have areas very far from the ocean, and there it can get very cold. Down to -50C in some cases, and regularly -30C or colder. I imagine heat pumps aren't used much there. But elsewhere (i.e. most places) they are great. In those places you see them absolutely everywhere now.
Maybe I don't understand what you mean by the word "feasible" – they don't have a goal of getting their living room above 23 C at most in winter, and I guess heat pumps are insufficient in such a house if you desire ambient temperatures above that. However, while other means of heating could plausibly bring the temperatures higher, that would end up being very expensive also because of the poor insulation – it's just harder in general to heat a drafty house and keep the temperature up, and I don't see how heat pumps are a uniquely bad choice for homes like that.
Edit: This is coastal Norway, so the climate in winter is quite similar to somewhere like Edinburgh, with temperatures usually above 0 C in January. The heat pumps would probably be insufficient somewhere the temperatures regularly reach -10 or -20 C, but that's a very infrequent event both here and in the UK.