Draughts mean that the hot air is escaping, which means any insulation there is being bypassed. You should try to fix that before you put the insulation in as you're more likely to just slow it to the point where it's hard to find but not fully stopped. Someone on here recommended Joss or incense sticks to see where the smoke goes.
Filling between the joists could be celotex or wool/rockwool, but celotex needs to be a firm fit, or sealed around the edges with expanding foam, which is going to be a nightmare if you've got limited access.
On the walls it'll probably be easiest using rockwool and then adding a sheet or two of celotex over the front or back to get the values higher. If it's on the warm side it reduces your room floor space, but also can act as a vapour barrier.
