So, I was looking on GAME today and I noticed the download version of the game was available to buy (it says "Buy" not "Preorder"), now i would try it, but i already preordered the game and have no use for another copy.
http://www.game.co.uk/en/mario-kart-8-299301?categoryIdentifier=683236&catGroupId=
I'm not 100% sure if it'll work or if you'll just get an error when you try to redeem the code. But if you're in the UK and looking for the game a day early, it wouldn't hurt to try this, I doubt you'll find it in physical form a day early.
http://www.game.co.uk/en/mario-kart-8-299301?categoryIdentifier=683236&catGroupId=
I'm not 100% sure if it'll work or if you'll just get an error when you try to redeem the code. But if you're in the UK and looking for the game a day early, it wouldn't hurt to try this, I doubt you'll find it in physical form a day early.