The FCO 'Know Before You Go' campaign gives essential information: see https://www.gov.uk/knowbeforeyougo
UK Embassies overseas: You and your family/friends need to know how to contact UK Embassies/High Commissions overseas.
Foreign Embassies & High Commissions in London: Contact these for visa arrangements for UK citizens going abroad.
FCO Travel Advice: Essential information to ensure that British travellers are well prepared before departure.
HM Passport Office: Providing information and online facilities for all aspects of application, renewal and amendments of passports for British nationals resident in the UK.