Have you looked at things like Evohome, Nest etc. - that would allow him to have his office at a much higher temperature than other rooms if desired, and run on its own timetable.
We use Evohome - it supports up to 12 zones; each with its own thermostat and timetable. It's great! So in the day our offices are heated, at breakfast/lunch the kitchen/eating area are heated, in the evening the lounge and kitchen, then just before bed the bedrooms get warm. You can also boost each area if you want to, on demand. We both work from home - in the room my DH works in he has it about 18 degrees, whereas mine is about 21/21.5 and I still get cold sometimes and boost it up to 22. The bedroom in the evening is about 18 and in the morning about 20. It's pretty flexible and was about £1.1k for all the bits (controller, TRVs etc. for a 4 bed house), plus a sparky to fit something for about £100.
That might be something to look at to help reduce the bill, so you aren't heating the entire house just to keep his office warm (on the assumption you aren't going to want to sit in a sauna, and he feels the cold). I suspect it would pay for itself relatively quickly, if you're currently heating the whole house when just using a couple of rooms during the day.
We also upgraded our windows for about £500/window to modern PVC double glazing about 2 years ago which made a big difference to heating (and cooling in the summer). And again - at £660/month it might net you big savings.
FWIW our combined elec/gas bill is about £120/month although going up as our provider went bust. That's 2 of us WFH all year (at the moment) in different rooms, electric underfloor heating in the en suite, heating on quite a lot of the year because it's a cool house facing N under trees.