Mission accomplished!
I can confirm they take UK games, they even scan the barcode but apparently the computer doesn't recognize the difference between GB and IT games (or it doesn't care) so it's all good. The girl at the counter didn't even check for discs, scratches, manuals or for the contents of the limited edition

(I gave them The Last Story LE, had half an idea to put it on ebay since I didn't like the game very much but this was much quicker given that it's not very sought after, at least in Italy). Now I know I'll never buy used game from Gamestop. Thanks a lot Cuddler, I don't usually check gamestop's website (nor do I go to the stores) and I would have lost this amazing offer if you hadn't posted about it. Cheers!