I spent ages yesterday shopping for some (any) C batteries for my kids new toys. Everywhere seemed out of stock so I went home and checked on-line. I found Argos were doing "buy one, get one free" on many Energizer Ultimate's so I reserved a few packets and just picked them up locally.
I believe Energizer Ultimate along with Duracell M3 are the most powerful batteries currently available.
BOGOF is available on the following.
Energizer Ultimate C Batteries - 2 Pack. £3.19
Energizer Ultimate 9V Battery - 1 Pack. £3.19
Energizer Ultimate D Batteries - 2 Pack. £3.19
Argos are also offering "3 for 2" on Energizer Ultra batteries.
The cheapest I can find elsewhere are £1.95 per pack of "C's" at Ebuyer, but you only get a single pack and you have to pay extra for delivery.
edit: Also AA and AAA have special 4+4 free packs for £3.19.