Title is self-explanatory. Cheapest I can find at the moment. Even cheaper than Amazon who were selling for £25.97!
PC version also cheap: http://www.hotukdeals.com/deals/portal-2-pc-14-99-game/950268
Apparently, this comes with FREE steam code as well! This sells for around £10 on eBay.
Stingy Tip: Don't forget 4% cashback.
Stingy Tip 2: CEX will buy this for £26 so if you buy more than 1, you can sell the rest for a tidy profit =P
Stingy Tip 3: You will also get 1000 game points - worth £2.50.
It's this price on X360 now as well!
EDIT: Xbox 360 version now also £22.99 and instock.