Tick 'No Religion' on the census form. 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. For instance, writing 'Jedi' is a wasted vote, because it will be counted as 'another religion'. For more information and further reasons to tick 'No Religion', visit The Census Campaign website.
If you don't have an active faith, please make sure the government knows. More info here