I wouldn't say it's huge popular, it has a cult following dedicated to going to games. Our sport culture helps. Supporting our own team on the other hand may not work, especially if the home games are branded the same as any other American team (cheerleaders and all that jazz)
Steelers don't have cheerleaders either. I have cousins in the UK who have been adopted into the Steelers Nation.
Think you guys too the cheerleading part too literally haha. Main point is, if you want a team to succeed in the UK, dropping a lot of the things associated primarily with American sports will go a long way.