There are some hard facts about this from the 2001 Census, published by the Office of National Statistics here:
Census 2001 – Ethnicity and religion in England and Wales
This week two surveys were published which also shed light on how well informed about religion some of us are. One was conducted by YouGov for Sky Box Office in advance of their TV premiere of The Passion of Christ:
Birthplace of Jesus A Mystery to Many Press Association
O Little Town of … where? Guardian
Jesus born in Bethlehem is news for many Telegraph
The second survey was conducted by GfK for the Wall Street Journal Europe and was reported by Ekklesia under the headline People believe more in God than religion suggests survey . For a more detailed report read this GfK page.