One of my neighbours live in an old cob cottage and they do just that.Their only heating was an old Rayburn and a wood burning stove. Last year they were persuaded that they could have a free air source heat pump. They had some insulation done but considering the depth of the walls and the general shape of the cottage this was only partial. They still have to run the Rayburn and the log burner but now have a substantial electricity bill
That sounds soo familiar. Thats exactly how I lived before moving back to UK - same sort of scenario with air source heat pumps. Most housing is so poorly built/insulated that there is no way a heat pump can provide anywhere near enough real heat. I have found that new built houses in UK are even worse than '60's built brick ones. I guess its all down to building codes being either totally inadequate or just ignored by the big builders - which is it I wonder?