The sites below carry current guidance on visas, passports, health and travel advice, so use them to confirm what applies to you.
www.homeoffice.gov.uk
www.fco.gov.uk
www.iatatravelcentre.com
As a rule, entering a country outside the EU calls for a visa, and a layover in a third country on a connecting flight can call for a transit visa. The country pages on the Foreign and Commonwealth Office website are a good place to check. That matters most for travellers who are not British, citizens of other EU states included.