Visas are NOT required to visit, live, work or study in the UK by anyone who lives in a country which is a member of the EEA (European Economic Area) and Switzerland.
The members of the EEA are the 27 members of the European Union plus Iceland, Lichtenstein and Norway. Croatia has provisional membership.
Outside of the EEA, the UK has complicated rules for visa entry and there are different rules for different countries. Citizens of the USA, Canada, Australia and New Zealand, for example, can visit the UK visa free for six months (but they still need a visa to work, study or live in the UK).
Citizens of most countries in Africa and Asia, however, need a visa.