Use the code GMG20-AWDF9-Q1PEY to get Borderlands 2 for £21.60. The code is valid until 25th September but it looks as though the pre-order bonuses are no longer available now the game is out.
Posted before but the price has reduced slightly and the code has changed. I think this is the cheapest it's been except for glitches. It's a Steamworks game (so activates on Steam) and the code will be displayed on screen as soon as payment is made. This is not a physical copy.
Don't forget to order through Quidco (6%) or Topcashback (6.06%) to get the price reduced even more.