Mario Kart 8 is fantastic by all accounts, and you can currently pre-order a physical copy for £36.85 from Gameseek. However, if you're specifically in the market for a download (despite the Wii U's hilariously limited onboard storage), CDKeys are retailing for a similar price with an extra discount if you 'like' their Facebook page.
Both versions qualify for Nintendo's amazing free game promotion, which lets you pick from an impressive list of downloads. Click here for details. Thanks to doglobster @ HUKD!