This is part of their instore 'up to half price' promotion.
Seemed to be for the PS3 version only, the 360 version was £12.99 like it is currently for both formats on the website.
They seemed to have at leas 3 or 4 copies in the store which I visited earlier today. I didn't buy it as I've had that game before and I lack the patience or time to dedicate to it, plus the idea of gigantic mutated cockraches, ants and rats terrifies the living shit out of me.
They also had Mirror's Edge (PS3) for £4.99 too.