I bought it from WH Smith in Manchester Piccadilly Train Station today and it's installing now.
I'll warn you though that PlayOnline is a pain in the arse to setup, it needs a hard drive, as it installs what seems to be the entire fucking dvd to the hd (I'm not kidding, the install takes 45 minutes), and the magazine estimates that beta users will be able to access the game free of charge for at least the next three months.
With regards to stock, they had loads of them in WH Smith and they were selling the mag in HMV up here last weekend, but one of my mates on Live who lives in London just told me that he hasn't been able to find it anywhere. I can only assume that the north received their supplies early or something.
There is also an ebay auction from a guy in Crawley who is flogging a copy. http://cgi.ebay.co.uk/OFFICIAL-XBOX-360-MA...1QQcmdZViewItem