According to this article on Sci Fi Wire, some 404,000 people in England, Scotland, and Wales listed their religion as "Jedi" on the 2001 British census (HT: Instapundit). The BBC reported on the growth of the Jedi movement in Britain back in 2003. The Jedi claim that they are Britain's fourth largest religion.
The Sci Fi Wire article also states that the British Office of National Statistics doesn't recognize the Jedi as an official religion and has lumped them in with the atheists. Counting the Jedi as atheists may be technically correct. Throughout the six Star Wars movies, there is no indication that the Jedi worship a God or gods of any kind. They do revere the Force. But the Force seems to be an impersonal power similar to gravity, rather than a conscious deity. Indeed, the ability to use the Force seems to be triggered by genetic anomalies rather than by any spiritual connection to supernatural entities. Jedi mysticism appears to be a secular moral philosophy like utilitarianism or Kantianism, not a religion.
As an atheist myself, perhaps I should welcome the rise of the Jedi in Britain. However, as sci fi writer David Brin documents here, here, and here, Jedi morality has many flaws. Even worse, the growth of the Jedi movement may cause the rise of the Sith, which Tyler Cowen claims is an inevitable consequence of the Jedis' authoritarian tendencies. Tyler is not as forgiving of the Jedis' flaws as I am. On the other hand, Bryan Caplan effectively uses public choice theory to defend the Jedi. Bryan makes some good points; but I find his lack of faith in the Galactic Republic disturbing. We'll just have to see if the Force is with Britain's new Jedi or not.