OK, people questioned the value of the 360 Limited Edition I posted earlier, try this one: Play-Asia are stocking the Xbox 360 version of the standard BioShock game for approx £25 after currency conversion.
It's listed as a Japanese game but the lead programmer on the game, Chris Kline, said here in the official forums
"The main version that supports EFIGS (english, french, italian, german, spanish) will be a worldwide release, so that version will run on machines worldwide."
There is more discussion ]here
So the Play-Asia version will work on UK Xboxes just fine. I and my friends have used Play-Asia many times in the past and never incurred any taxes. They also ship pretty quickly in my experience, although it seems doubtful you would get the game launch day. If you are concerned about the language used in the manual etc they also list the US version for approx £29.