First post, apolgies if it has already been posted. It also seems to work for Mirror's Edge and Velvet Assassin, but this game seemed to be the most appealing.
This is similair to the Argos deal earlier in August - I've tried it for 360, not sure if it works for PS3. Add BioShock to your basket, and then add another £17.59 game. It should show up as £12.41. Go through to reserve store pick up, and delete the other game. It should still be £12.41. :)
EDIT: Also works for Resistance 2.
EDIT: Why the coldness?