I think it's becuase Amazon have sold out of it, and the £54.99 is from a Marketplace seller. Currently it's being sold by Express Games. Once it comes back into stock, I'm sure it will drop down to around the £30 mark again. I luckily managed to pick it up last week for this price, but hopefully it won't become another wii fit where you can only get it in bundles of games you don't want!!
I am shocked at the price here on Amazon. The RRP is £29.99. In America it is only $29.99. Just because no-one else has it in stock is no reason to rip people off so badly. I for one am voting with my mouse and I'll wait until it comes back into the shops when the hysteria has subsided.
I found this game on Gamestation for £29.99 & ordered it straightaway, looks like it's out of stock with them too now. I agree that £54.99 is a lot here, but keep looking & hopefully it'll be back in stock soon :)