I shall be there rocking my #39 jersey, my tickets arrived last week
I went a few years back to the Saints Chargers shoot-out and what I did was drive from Bristol to a subway station in watford, parked in a nearby quiet residential street and got the underground to Wembley and back. I haven't decided what station to drive to yet but i'll be doing the same thing and driving home after. Can't give any travel advice into London though, I don't know it well enough but I know from experience after the game Wembley way is choc a block with people making there way to Wembley Park station and it takes quite a while if memory serves me correct to even get to the station but it is constantly moving so not terrible. Whatever you do after it is going to be real busy, I'll just stay in my seat taking photos and stuff until I get kicked out to allow the crowds to disperse a little. There is a train station around the back of the stadium which may be of more use to you, you'd have to check the routes though.
As for prior to kickoff, there is a "tailgate" which isn't really anything like an American tailgate but is just an area with food, drink, merchandise and a little NFL related entertainment i.e. CHEERLEADERS! (hopefully). It's not great by any means but it's not bad either. That opens at 11am and the queue gets quite large to get in but they don't close the queue until 3.30 so there is plenty of time.
Getting to Wembley will be easy as everyone arrives in drips so I wouldn't worry about being late, it will take a while to leave though. I plan to get there around 11-12 and Win or lose it is going to be awesome.
Would be great to have a beer with any SML-ians attending.