I think they can work but they are eye wateringly expensive to purchase and the point about the radiators having to be twice as big is well made. A friend of a friend has a heat pump but he spent north of £40,000 on his house getting the insulation to a state of the art condition, heat exchangers to minimise losses through ventilation and the pump itself. His energy bills dropped like a stone but even he admits he won't live long enough to break even.