Just enter the code SEGCON12 to take off the 20%.
Not worth it for every game as the prices aren't too hot but definitely on a handful of others it can knock off a sizeable chunk of money, especially if you pre-order a few. Code is one time use only I believe.
Brings the following down plus a lot of others:
Pokemon Black/White 2 - £20
Borderlands 2 - £26.39
Transformers FoC - £30.39