If you're looking for a cheap and cheerful insulating option then lagging your tanks, pipes, and radiators is a great place to start. The cost is little to nothing at roughly £15 for the tank and £10 for the pipes. The best part is, lagging your radiators and tanks will pay for itself in 6 short months!
If you have a bigger budget and need a bigger impact, go for the walls. It is expensive: insulating the external walls will cost you £11000, and for the internal walls £7000 but you can be sure that it will let no heat out!
If your budget isn't big enough to afford a wall insulation, consider the next best thing: cavity wall insulation which only costs £500 and only takes 3.5 years to pay for itself but will save you £140!
Last but not least changing your floors to timber flooring will cost you £770 and pay for itself in 2-10 years saving you £60 on your yearly bill!