We have just completed the renovation of a large Victorian house. As it's a sell on, or rental in the current market, we had a budget to stick to.
However, we used polystyrene veneer on the inside of most external walls. Mr R has applied this before at work, on a job spec. It's cheap, under �3 a roll. Apply like wallpaper. Read the pack for suggested paste. We then lined on top and painted. I have no idea of stats on it's effectiveness, but I think it helps, the wall is warmer to touch. It also hides slight imperfections in old plaster, if you are not removing it all. It won't respond well to knocks tho.
We have double isulated the loft, a large one, with 2 layers of the thicker depth insulation, now making it about 16 - 18" deep up there. If yours needs doing, get yourself to Wickes, it's on a buy one get one free at the mo. Saves lots of ����s.
Just had a great result from our energy assessor.