I just spoke to a very nice man from the Hampstead O2 store.
He confirmed what has already been said in this thread - that O2 have changed their policy to allow in store upgrades. I also asked about whether they will have white iPhones in stock, and he told me that as far as he's aware they will do. Fingers crossed on that front, then.
Also, they're opening at 8:02.
Well in a way I'm sort of glad that i don't think i got an order through as there is something rather fun about waiting in line for a few hours to be the first to get my hands on an iPhone in my area!
For the original release i waited for 4 1/2 hours to be the first one!