I know that this has been posted before, but the last one was 5 months ago, and with Christmas coming up it is only going to get even harder to find these. Gamestop in Birmingham have these still in stock. Not only that they are still charging the RRP- that is £69.99. I got mine yesterday and yes it is a brand new unopened Wii Fit, not a second hand one. I'm not sure exactly how many are in stock, but it seemed like quite a few. The shop is located on the walkway between the Pallasades and the Bullring (on the upper level, past Debenhams- http://www.bullring.co.uk/website/CentreMap.aspx)
Obviously this is only helpful for Brummies- but there are apparently Gamestop shops up and down the country. The company is apparently quite big in America, so they are not an independant store. Unfortunately they don't have a website for the UK, only the US. So any orders from that website will probably incure import tax and take ages to reach the UK (the same goes for all online shops situated in non-Eu countries).
Another tip is this site; http://www.wii-consoles.co.uk/wii-fit-in-stock.asp.
It keeps a track on which online sites have stock and at what price. Also, HMV at the moment are doing a notiification service, that will e-mail you as soon as they have more Wii Fit's online.
Good luck to all those WiiFit hunters out there!