+1 for advice from matt_outandabout
Doing the loft yourself is easy. Just wear gloves and long sleeves if you're using glass fibre rolls. And make sure that you shift the wiring for lighting to the cold side (there should be enough slack to do this). Also its important that the loft hatch is well draft proofed as well.
Cavity walls are a bit harder. Fine for some properties, not so good for others. Depends on brick porosity, mortar type, exposure of the house. Might be worth finding out if your neighbours have had it done and if they've had any probs. I wouldn't trust the advice of a cavity wall insulation fitting company on this one… Having said that my mate had his done and it works really really well, with a noticeable increase in temperature, i.e. much more comfortable house.
Energy suppliers are obliged through CERT to reduce set quotas of CO2 through insulation measures.
This website might be useful.