On a rather cynical note - reduce the room temperature to 15C and you should see a considerable reduction in the problems that are occuring, whether or not you could live with 15C is another matter.
On the assumption that the majority of people would suffer at 15C then the only option available would be to introduce internal insulation to the walls and if your really keen, dig up the floors and put in an insulated floor slab (you will be amazed at the improvements in comfort levels, as well as reduced fuel bills, not to mention what the energy reduction will do for your contribution towards the carbon initiative - reduced carbon and CO2 emissions!) - a win, win situation or should that be solution!
Going back to the wall insulation, use a PIR sandwich board thermalite super or similar - when you come to fitting it make sure you leave a minimum cavity of 25mm between the existing wall and the back of the insulation. Best way of doing this is to use some 25mm thick bitumen impregnated board cut into say 100 x 150 and stick them onto the wall at 400 centres horizontally and say 600 centres vertically (same adhesive as recommended by the manufacturer for the wall board). Once set, apply adhesive to these dab's and stick the insulated plasterboard in place, which will give you your cavity, which has the added benefit of giving you the opportunity of being able to run new electrical cables in the cavity, rather than having to revert to surface mounted socket outlets (or whatever). Whatever you do, do not use mechanical fixings which would go through the impregnated bitumen board.
One point, not that I am trying to tell anyone how to suck eggs! - you only need to insulate the external walls.
Regards