Yeah London can pull a crowd for an international, but there's a big difference between internationals and a club game between two teams from the opposite side of the world that most of the population's never heard of.
Sure the NFL pulls good crowds, hell there'd probably already be at least one NFL side in England by now if they didn't have to worry about competition balancing issues and other factors, but the NFL is a powerhouse with cultural relevance that the NRL simply can't compete with.
Don't get me wrong, I'm not saying that NRL matches in England, or the UK more broadly, couldn't or wouldn't work, in fact I think they could be reasonably successful if you organised and marketed them well, but it wouldn't be the sure thing that many seem to think either.