You'll find everything you need to know at these links:
Heres the British American Football Association:
http://www.bafanl.co.uk/
And heres the British Universities American Football Association
http://www.buafl.net/
To the best of my knowledge, they play using BAFA rules which you'll find on the website - I'm not too clued up on the intricacies.
But bear in mind that here all of the best athletes are recruited to rugby/football (soccer)/athletics.
Also, the leagues here are completely amateur (Although I believe some of the big London teams, such as the Blitz or Warriors, have had past NFL scrubs playing for them - ie Bradlee Van Pelt played here I believe).
But the important thing to note is: We have enough people here paying their own hard earned money to be able to play american football, both at a university level and a league level - because this is the only way the leagues stay afloat, the majority of their income is the membership fees from the players.
We have enough people playing to have 2 tiers (Premiership & National) and various divisions throughout the country. If the NFL REALLY want to grow the sport, this is where it has to start. There's no point getting people in their 20s, 30s, 40s + interested by putting on Wembley games, they need to get children involved from an early age.