Use code 15WYS10-1 (thanks to Millercat for that).
I still think this is too expensive for this game, but only 5p more than the Sainsbury's deal and you don't have to be a new customer, and it's in stock for next day dispatch.
Quidco available brings it in at under £30.
£36.99 at Amazon.
Xbox link: http://www.tescoentertainment.com/store/games/-portal-2/8%3a736598/