Saw these today (Sainsbury's London, EC3M) - 19p cheaper buying in singles than in packs, and of course you get a variation of flavours. Not sure it's the best deal, but very cheap for a quick snack.
They're also doing Kelloggs Special K Bars for 28p each! That's 3 calories for each penny! (I am obsessed).
Please don't slay me lol, I don't put up deals often!
Not available online as far as i can see, and I can't be sure if its nationwide.