Snuggling up to an electric aga is a warming experience!
I had the very pleasant experience of being reacquainted with an Aga stove when i was visiting relatives for a house-warming recently. Their particular stove is retrofitted to run off main electricity, acting as a heat store for cheap Economy 7 power at night. The large thermal mass acts as heat storage right through the day and is still hot enough to cook on and in by the evening.
While winter hasn't yet set in here this stored heat was all they had in the whole building and it made a considerable difference to the feel of the whole place. The building itself is an old farmhouse, however it has been modernised with high quality double glazing, well fitting doors and decent insulation. The whole place was remarkably comfortable and they haven't even got furniture in yet!
I grew up in a house with a gas, rather than electric Aga in the kitchen which provided a big part of the heating for the whole building - it was always lovely in winter to come home and lean against it to warm up.
While electric agas (even those using Economy 7 rates) are not likely to be cost effective this is surely one of the best demonstrations possible of thermal mass in action. These days Agas, and other commercial cooking ranges, come in a wide variety of models, possible fuels and options. Gas and electric versions are obviously the most convenient, but wood can be a viable option if you live off grid and don't mind fuelling it regularly.
Remember that a good quality heat storage stove - whether in the kitchen or sitting room - will provide a large portion of the heat for your home, and can be some of the most efficient heating solutions available.