The English and Welsh Census asked the question “What is your religion?” with 14.81% selecting "None".
The Scottish Census asked the question “What religion, religious denomination or body do you belong to?" with 27.55% selecting "None".
Either way, all other polls* indicate that between 40%-53% state "No Religion".
One can only hypothesise that reason for this is that the census questionnaires would generally have been completed by one person per household, most probably the oldest member, the head, such as a parent. The questionnaire would have been completed on behalf of children, babies, adult 'children' still at home and adults with special needs, and by default, most would have been selected as being "Christian".
All recent opinion polls indicate that the majority of those aged over 50 claim to belong to a religion with less than a quarter of those aged under 25 claiming the same. The 2001 census showed that 26% of the UK population were aged under 25 with 19% being aged 45-59. Clearly, those completing the census questionnaires were more likely to be 'older'and more likely to be 'Christian'.
*The only other recent document to indicate a figure of "No Religion" to be contrary to that of other recent polls is the one on Prison Statistics giving a figure of 32%.