There was actually a scheme a few years ago to get cavity wall insulation for free. I know because I took advantage of it, although the work was not exactly first class.

Insulating the loft ought to be cheap because it's simple and material is cheap. I DIY'd it.

On the other hand, insulating the floor is a b*tch. I did it myself as well and lifting the floorboards of the whole ground-floor was a massive pain but now I have 10cm of Celotex underneath and enjoy warm wooden floor all year long with no draft.