If you are already a member at a local coop store you might have noticed recently they have been printing out vouchers for 10% money off your next basket amounting to £20/£30/£40/£50 for people in their membership scheme.
It might seem like a ploy to make you spend more money in the shop to reach that £40/£50 mark, but its often overlooked on the ticket that it states the voucher works on anything apart from petrol. Therefore by this you can actually reduce your electricity/water/gas bills (at the most expensive side of the year) by just going to the coop with that (vacant and dusty) old membership card.
Sidenote for anyone not a member: In my local store people who have not wanted the higher value coupons have been leaving them around the till area, so you may even not need a card in such a case.
Warning: Lurking around other customers to get their unwanted vouchers may make them think of you as a cheap voucher hording whore. Congrats.