A relatively convoluted process to get this pre-order price. Firstly, you'll need Virgin's £5 off at Game.co.uk voucher. I'll reproduce the info on how to get that here, for easiness' sake. Go here:http://virginmobile.crave-exclusives.co.uk/bdr-emp2/virgin/details.xhtml?offerId=12
Follow the link, complete the details with fake information (or real if you want). An email address is not needed so you can make that up as well. On the next page it will tell you that you need to register. Click Register, then go back a page and refresh. You'll see a code on that page now. That'll entitle you to £5 off any purchase over £30 at Game.
Now go to Game (via Quidco if you want the extra 11% discount). Add BioShock to your basket (£29.99) and search for gift wrap and add that as well (99p). That'll take the total order over £30. Enter the code you got at the Virgin website, taking it down to £25.98. Purchase.
Now find your order screen and you should notice the £5 has been taken fully off the BioShock order and it reads as having been pre-ordered for £24.99. Cancel the gift wrap order immediately (you have a wee while before they debit the 99p), or keep it if you really want/need it.
Hey presto. BioShock for £24.99, or £22.41 with Quidco.