Included in the Boots 3 for 2 items offer, you can buy Duracell batteries. At the moment, Duracell are running their own offer which is 4 + 4 free on both AA and AAA Duracell Plus batteries.
Priced at £3.95 a pack, this means you can get 24 batteries for £7.90.
I cant see the Duracell Plus ones on the Boots website, so putting this down as Offline only.
Alternatively if you dont want that meany, you can pick up 1 pack for £2.99 in Argos (Where the image is linked from)