Scan's today only page has a few Wii games listed, the most notable of which is WWE SmackDown vs. Raw 2008 for £12.91. Using the AVForums free delivery offer, that's easily the cheapest I've seen it going for lately. Find-Games.co.uk has the next cheapest as Argos at £19.99 + delivery.
It might not be the greatest version of the game but I've played worse!
** Note that being a today only special, searching for the game on scan will find the 'normal' price of £14.91. Buy directly from the page to get the price mentioned.