The way to work this roughly would be assume 20% of your heat losses are through the roof and assume your improvements could halve this, so take 10% of your annual heating bill of, say, £2,000 and with a 15 year payback this would allow capital expenditure of £3,000.
This is the minimum BC would be looking at for part L improvements. They may allow you to make the improvements elsewhere if they are not possible in the room you are working on.