So, according to the Office of National Statistics:

About sixteen per cent of the UK population stated that they had no religion. This category included agnostics, atheists, heathens and those who wrote Jedi Knight.
Office of National Statistics 2001 Religon Statistics

Note: “agnostics, atheists, heathens and those who wrote Jedi Knight” heathens don’t exist. This irritates me, esspecially since I’m at a pagen wedding next week, it annoys me that the ONS are quite so dismissive.