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?