I'll second that I needed one for my wii sisters b-day (see what i did there!) and Virgin in Dundee and Perth had quite a lot in stock yesterday. The usual suspects, GAME, Gamestation etc didn't.
I'd also try places like HMV, Woolies, PC World, Toys 'r us if I were you...
John Lewis High Wycombe has a load of Wii Play maybe 15 copies a couple of days ago and nothing else Wii related, so I would try other John Lewis stores too...