I'm gonna head there for around that time, but I live three miles from Twickenham, it's my birthday and my mates will be there with beers (not too many, I'm not an idiot) and chairs and maybe disposable board games so I'm a bit weird. I heard from someone at Seattle they got there for 4pm and got a good spot just a couple of rows in front of Edge, but I'm a bit paranoid.
My guess would be it's a popular gig, so I think around noon would be a safe bet - go early if you're uber keen. What I wouldn't do is panic based on the US shows - it always seems tense outside the stadium, but once you're in, there's always quite a lot more space towards the front then you think.
I'll probably rock up around 9/10, but I'm a little weird. My mates will probably detest me by the end of the day. See you then maybe! And if you do go early - Fry up beforehand.