1 Month, 3 Weeks, 5 Days. Census day is 27 March 2011.

Sample forms/questions/etc:

England & Wales - http://tinyurl.com/ygvd5e4
Scotland - http://tinyurl.com/6gu4qou
Northern Ireland - http://tinyurl.com/6fq7khd
Republic of Ireland (April 10th) - http://tinyurl.com/5vt6z2d

"In 2001 [...] 390,000 people responded "Jedi", leading it to be reported as the fourth-largest* religion in the UK. Despite this, it will not be listed as one of the tick-box religion options on the 2011 census."
http://www.independent.co.uk/news/uk/home-news/out-for-the-count-could-the-2011-census-be-the-last-2180194.html
*Christian: 70.0% / No religion: 14.7% / No response: 7.8% / Muslim: 3.1% / Hindu: 2.1% / Jedi: 1.79%

I was Jedi last time, but I've lapsed and I've since converted to the much more indie Church Of The Flying Spaghetti Monster, so I'm a Pastafarian now.