You haven’t said what tarrif you are on, but here’s some numbers…….
My house – 3bed bungalow, heating on 6-8am, 3.30-9.30pm weekdays, 8-9 weekends. In December we used 2500kwh of gas CH.
If that were electric at a standard rate (approx 14p/kWh) it would have cost £350.
If you are on a wet electric CH then I would expect you to be on a Better than standard tarrif but I don’t know how cheap it can go.
You need to check your real meter readings, and check what tarrif you are on to be able,to make any meaningful comment.
Edit: spotted that it is £200 not £300 – that’s not too bad for heating/domestic usage in December.