Roof insulation is one of the first items to fix. Get R5 or R6 earthwool from Bunnings. It costs a lot but fuck is it good. It’s actually very easy to install, just be careful around the downlights (you need a gap around them).

The other quick fix is all the drafts cracks and leaks. Get door snakes, door and window seals, blinds. Don’t worry about the floor heat won’t sap from that.