Pack of eight cans of coke (all varieties) are £3.29 at Morrisons; with a "buy one get one free" offer on at present.
Works out at a shade over 20.5p per can. We've seen similarly cheap multipack deals for coke at several retailers recently, but this seems to be the cheapest on offer at the moment by a long shot.
Can never be sure with Morrisons whether it's nationwide, but it usually is when it's groceries. (It seems they regularly have special offers on electricals etc. that are store specific.)
I've confirmed this price in two shops today, though. (Guisborough and Hartlepool.)
That's me sorted for my diet coke fix for a couple more weeks... ;)