Get rid of gas. I did one year ago exactly today and I've only used 2300kWh of electricity since.
That is a massive expenditure.
Our combi is gas of course so it would have to be air source replacement. The radiators are connected to microbore which could be a problem.
Our kitchen is built around a gas aga.
The gas aga isn't that much of a problem with gas consumption as a whole, in normal times, the amount of heat it gives off in a 20 year old insulated house means the gas boiler doesn't have to work too hard.
We do however have to turn it off come spring time because of the heat, and why as a result our electricity usage is constant throughout the year, as we swop from winter to summer.
Strangely comparing gas usage with friends with similar houses, they use twice as much gas, probably due to old inefficient boilers, which they are shocked, "but you have an aga"!