For UK and England and Wales:

Office for National Statistics (ONS)

Census Customer Services:

Tel: +44 (0)1329 444972 (Phone)
9am to 4pm Monday to Friday
Tel: +44 (0)1633 652981 (Fax)

Email: census.customerservices@ons.gov.uk

Postal address: Room 4300S, ONS, Segensworth Road, Titchfield, Hampshire, PO15 5RR

Websites:

UK Censuses homepage

2011 Census homepage, England and Wales

For Northern Ireland:

Northern Ireland Statistics and Research Agency (NISRA)

Tel: 028 9034 8160
Email: census.nisra@dfpni.gov.uk
Website: www.nisranew.nisra.gov.uk/census

For Scotland:

National Records of Scotland (NRS)

Tel: 0131 314 4254
Email: customer@gro-scotland.gsi.gov.uk
Website: www.gro-scotland.gov.uk/census