Game are currently doing the offer where if you trade in any 2 Wii game you get Mario Kart for £9.99. This offer is shown instore and in there trade in guides.
If you go to choicesuk you can get Cosmic Family on wii for £7.99. Get two of them.
No delivery charges as postage is free.
Once received you could have a play on it. Then trade for mario kart.
This seems to be the cheapest game around.
And certainly the cheapest price for mario kart ive seen.