This item is listed as "DEAL OF THE DAY" in GAME's promotion and they have very limited quantities available, so act quick to receive the game at half price.
All you have to do is purchase the game before they're out of stock of a limited quantity, so be quick.
Then use the voucher posted by gary_rip and the code for the £5.00 voucher is (J3PW74) which entitles you to a discount for a £30.00 or more purchase, so you will also require to order something for at least a penny to get £5.00 off and as gary_rip once again suggested, you should order AA batteries for £0.98p in order to bump up the price :)
Hope a lot of people find this useful and remember that GAME are doing this promotion on a daily basis for different items.
P.S Don't forget to go through quidco to receive 5% cashback, which is about £1.50 to bring the game and batteries to less than £24.50.
P.P.S please remember to vote this deal hot and give rep if you like my findings :)