The UK Data Service Census Support service is responsible for online registration (via EASE) for access to the UK Census datasets by the academic community. The site is a one-stop registration site for contemporary decennial Census datasets (1971-2011):
For some interesting visualisations of the 2011 census data for England and Wales see datashine.org.uk.
Information about the 2011 census including the 2011 Census programme, Questionnaire content, the census Coverage survey, census consultations are available on the ONS 2011 census website.
Registration is required in order to gain access to the ESRC census programme portals.