The UK Census is conducted and made available by the Office for National Statistics (ONS). Census data is also made available to academic institutions via the UK Data Service. If you require data for use with GIS software you should use the UK Data Service.
Histpop makes historical census reports available from 1801-1937.
Inter-governmental organisations (IGOs) including the United Nations, World Bank, IMF, World Health Organization, International Labour Organisation, OECD and EU are major providers of statistical data. They all make data freely available on their websites but may charge for some datasets.
Passport provides IGO data in a form that allows you to compare countries. The UK Data Service also provides access to some IGO data which is not freely available.
Most national governments have statistical agencies and many government departments also collect and publish statistics. Browse the University of Auckland’s OFFSTATS 'By Country' to find national statistical agencies. To find government departments, select a country and then click on ‘Government’ in WorldLII.