100 Extra Long Life batteries (either AA or AAA) only £14.99 - works out at approx 15p per battery. This is about 1/3 or 1/4 of the price of Duracell Plus (roughly equivalent according to Which?).
Come in a cardboard box and shrink-wrapped in strips of 10.
Available both online and instore (£2.99 delivery charge for orders under £35).
Obviously these are not rechargeable batteries (which are better for the environment) - but there is a large start-up cost associated with rechargeables and £14.99 won't get many such batteries! So please vote on the deal, not on your preference in this regard...