Not too sure that the atmospheres are that different tbh. I've been to Wembley 3 times for footie and my team has whipped up an awesome atmosphere (Crystal Palace - Playoff Final and the Semi and Final of the FA cup last season. I've sat in all 3 tiers at Wembley and only the upper tier feels too far away and is lacking in atmosphere; the lower bowl is fantastic though. Obviously England games are going to have a terrible atmosphere as we haven't exactly been given anything to write home about and also the teams we've faced in qualifying have been terrible. I've been to Twickenham too to watch England rugby Internationals and there are LONG periods of near silence. Lastly I've gone to two NFL games at Wembley and the atmosphere has been terrible not because of the Stadium but just because of the fans; there are fans from every team there so usually never get a great atmosphere as most of the crowd doesn't care if the Jags etc score (I didn't); I hate to say it but the Squeelers had a massive following inside Wembley for their game and the atmosphere was good.
If it is the Ravens that are coming to London then I can't wait, we seem to have a sneaky big following over here and Wembley will definitely be purple that day. However everything just points to the NFL going with the Seahawks - they fulfil the same criteria that the Ravens do i.e. never played in London PLUS, and this is the tipping point, they're scrapping the 1pm UK time slot so Pacific coast teams fans have a friendlier viewing time back in the States.