You can have it done free by British Gas. It's either cavity wall or loft insulation (not both) but you have to be an existing dual fuel customer if i remember correctly?
A friend of mine is also in charge of an incentive which will start on Monday for people who have Homecare agreements with British Gas (like an insurance cover incase your boiler breaks down!). As long as the person in question has a Homecare agreement but has their energy supplied by someone else, as long as they bring both the gas and electric over then they will get £200. The first £100 is given 3 months after supply and the rest is given after 12 months. Hell, you could take out a basic cover, do that and then still net about £80 or so.
Most energy wastage isn't purely from heat loss, most is from poor understanding. I know both EON and British Gas give their customers free electricity usage monitors so you can keep an eye on your usage on an easy to follow display that puts it in monetary value. It's all about education.
Also, people often go on about cavity wall and loft insulation but 99% of them over-look the heat passing between floors. When decorating any rooms, get your floor boards up and pack that up with insulation too. You can get new-age insulation that is about 10mm thick that does the same job as 300+ mm of normal roll-out stuff. Always get the better stuff (it goes off R ratings). Also, ALWAYS get the best underlay you can. Even though heat rises the floor will leech a lot of energy and again it helps to prevent loss between ground and first floors.
Also, insulate any unseen hot water pipework as it improves the systems efficiency. Make sure the system is balanced and you have the right size rads for the rooms (type BTU calculator into google). Get the reflective backings for behind the rads to reduce heat loss via the walls. Heavy curtains help and make sure you tuck them behind the radiators.
You can get insulate wallpaper and paints as well nowadays. It all adds up.
Thermal mass also helps, so attics filled with clutter, heavily furnished rooms etc as the furniture acts like a giant storage heater, releasing that energy as the temperature falls.
Insulative render, heated glazing etc. Christ, you could go on forever.
It is worth noting though that passive houses can suffer really badly from condensation so the more you insulate the house to reduce heat loss and movement of air the more you may suffer from condensation as the rooms don't get purged.