In the 2001 census, almost 400,000 people in England and Wales (along with over 150,000 other members across the British Commonwealth) listed their religion as "Jedi." Now one group is fighting back against this reporting—and it's not conservative Christians. It's atheists.
The site You Are Not a Jedi explains why the campaign is important. (And if you really want to know more about their rationale, keep refreshing the homepage and mouse over the main box.)
The data gathered is used to inform government policy, and was used by the last government to justify funding of religious community bodies over secular ones. For example, 2001 census data has been used repeatedly to justify an increase in the number of state maintained faith schools and the increasing level of government money spent on faith organisations.
By ticking 'No Religion', you will ensure that the Government receives an unambiguous message about the number of non-religious people in the UK. Any other response may be manipulated into a response in favour of religion and publically funded religious organisations.
Wikipedia's entry for the 2001 Jedi listings is called the Jedi Census Phenomenon. The site suggests that Commonwealth citizens self-reported Jedi as a religion either to mock their government or to protest against religious questions on the census.
What Wikipedia has failed to take into account: Maybe these people believe that there should be Jedi.