This is the cheapest I have seen a new controller inside a shop!
I did a quick search from a few places online to compare it to:
Amazon - £25 delivered
Play - £34.99 (Or available from seller with 84% positive for £27.99)
Tesco online - £25.57
Game/gamestation - £29.99
Shopto - £22.85
On a couple of the websites mentioned the controllers pictured are the black/grey ones, I don't know if that's important to people but I personally think the all black newer ones go much better with the slims - this is what the tesco in store controller was :-).
However if your not bothered about the colouring and want to wait for delivery shopto is 5p cheaper.
I'm not sure if this is nationwide but would imagine it would be as my local store (Redcar) doesn't normally have any special offers!