Over the years a vast range of official reports and statistics have been produced from the census. There is an Overview Unit on Census Outputs but the titles of the main census reports usually begin: Census of England and Wales (from 1861); Census of Great Britain (from 1801); Census of Scotland (from 1861). For Northern Ireland see Census of Ireland, Census of Population of Northern Ireland and Census of Northern Ireland. The majority of these reports come in multiple volumes covering different aspects of the census such as population numbers, ages, housing, health, occupations, marital status etc. Most local libraries will have recent census reports and complete census reports dating back to the first census will usually be found in your university library or central library.
Following the 2001 census a rolling programme of publications is underway. For England and Wales these are available online at: http://www.statistics.gov.uk/census2001/default.asp.
Statistics from the 1991 England and Wales census can be found here: http://www.statistics.gov.uk/census2001/accesscensus.asp
Results from the Scottish 2001 census can be found here: http://www.scrol.gov.uk/scrol/common/home.jsp
Statistics from the 2001 census in Northern Ireland are available here: http://www.nisra.gov.uk/census/start.html