First post, so be kind :)
Okay, now this involves a bit of traveling, but its worth it for this game.
First, go to GameStation, buy Mass Effect for £19.98, then go to your nearest CEX store, and they will buy it for £30.00 cash, then go back to GameStation to buy it once more. this might take a bit of time depending on how close these to stores are, but in Liverpool, its only about a 10 minute walk down Church street to the end then to Liverpool Central station.
so either way you look at it, you've just got an extra £10 Christmas money - or you just got £10 off the game. enjoy playing :)