Since you'll be working at/from home it's not unreasonable to ask
you employer to chip in for the extra heating costs. Open the bidding
at (say) 5 quid a week and be willing to settle for 2.
Also tell them that keeping office equipment on for 40 hours a week
will cost you money, and you'd like that back as well, please.

I'd guess that the house is otherwise occupied, so the amount of cool-
down during the day shoudn't be that high. You might even be able to
get by with just putting on a pullover and keeping the doors closed.