Find My Past – https://www.findmypast.com/
Contains passenger lists featuring millions of people leaving the UK for destinations worldwide.

British National Archives – https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/census-records/
Contains the historical censuses from 1841 to 1921 and provides information on using the census returns of England; Wales; the Channel Islands; the Isle of Man; vessels in English and Welsh ports and inland waterways from 1861 onwards; the British Army overseas (1911 and 1921 only); RAF bases overseas (1921 only).

FreeUK Genealogy – https://www.freeukgenealogy.org.uk/
Free UK Genealogy provides free online access to family history records. Specifically, birth, marriage, death records; census data, from 1841 to 1891, and parish registers.