Oooh, MD, much prettier than my idea! I was going to suggest canite panels. Sorry, not sure on the spelling, but its a fibre board that often gets used for pin boards. Cheaper than replastering (and putting in proper insulation) if you needed a stopgap. To pretty them up you could cover them in a nice fabric and even frame them (and just put a new fabric cover on for a change). Attach to the walls as large panels.

I think one of the underfloor insulations is called expol, and it fits up between the bearers, right under the boards. Its not like batting or anything, its very compact, and helps keep the floor from being too cold (which growing up in an old house with boards, would have been heaven!).