I tend to use Amazon in the UK. I think it usually gives you some sort of bonus (got to the Uncharted 2 beta through it) and ships very quickly (if you actually buy Amazon's offer and not someone else's at the site). I don't know about the price, but they tend to be competetive.
You could maybe also wait for launch day. In Amazon if you really want to get it on launch date you might have to use a fast delivery, which might set you back a few pounds. Otherwise you have to wait around 1-2 days. Remember how they released MW2 at Sainsbury's for fking 25 pounds when the retail price was 60? Maybe that'll happen again, one can always hope I guess.