As I said, the limiting factor on air-sourced reverse-cycle air con for heating is when the outside temperature is so low that the external heat exchanger freezes. In the UK I would expect you'd need some additional heating system for the coldest parts of the year (September to May in Cumbria if I remember correctly). In Perth (The Australian one rather than the original) we had a few occasions when the heat exchanger froze but but not many. A well insulated house obviously makes a big difference. It's been a while, but I think that unit used 4kW of electricity for the equivalent of 14kW of heating.