For UK nationals a passport is not strictly necessary but photographic identification is required. Check with your airline or ferry company as requirements vary. Other nationals – a valid passport or national identity card is required. (Passports are not required for travel between Northern Ireland and the Republic of Ireland).
For citizens from Europe and most other western countries, visas are not usually required.
Further information is available from the British Embassy in your country of origin.
Phone +44 (0)870 606 7766 or visit https://www.gov.uk/government/organisations/uk-visas-and-immigration
See here for further information on visa requirements.