Brita Classic 12 Water Filter Cartridges, £23.76 delivered at Amazon, works out at £1.98 each delivered, if you buy 12.
BRITA Classic 9+3 pack water filter cartridges
Removal of Chlorine, Limescale and other impurities
Fits all BRITA Classic water filter systems
One cartridge provides one months supply of BRITA filtered water
Suitable for all water types
These are the original Brita cartridges, not the Maxtra ones. They have been on amazon for a low price of £25.88 for the past few weeks, but I noticed this price drop today to £23.76 delivered for 12, which makes them under £2 each delivered for a year's supply (12 cartridges).
No quidco for Amazon, but, there is a 5% discount for NUS card holder's.