I'm in the process of buying a house with an F for energy efficiency (exact same score, actually!), it's about standard for croft houses needing modernisation. The last one we bought, we took back to bare stone walls, insulated the walls and roof properly, put in underfloor heating and replaced the failed double-glazing. It now keeps itself beautifully warm and 750 litres of oil (£350-ish) lasts about 9 months.