ahhh, the joy's of english law..
Take the game back and explain that it does not work because of a CD key problem, simply explain that and ask for a replacment..
They can not refuse you, the product has failed to operate as intended..
http://www.tradingstandards.gov.uk/
Lot's of info there if you look for it, but you don't need to go down that road, just take it back and tell them what's wrong so they don't end up reselling it..
Also might be an idea to get in touch with whoevers running EQ2, and tell them you where sold a boxed game who's CD key is in use, so they know somthing's not right with a player..