Familyrecords.gov.uk is the official source for family records in the UK, so is a good place to start.

National Archives is the national archive of England, Wales and the United Kingdom. They look after national archives including births, deaths, marriages, military records etc. The organisation runs an enquiries service through their website.

Scots Origins is the official government source of genealogical data for Scotland, in association with the General Register for Scotland. You can search the 1901 Census online (for a small charge).

For Northern Ireland try the Public Record Office.