Entry requirements for US citizens
US citizens need a passport and a UK Electronic Travel Authorisation (ETA) for visits of up to 6 months.
The ETA costs £20, is valid for two years or until your passport expires, and allows multiple trips. Apply through the UK ETA app or on GOV.UK; decisions usually arrive within a day but can take up to three working days. Your passport must be valid for your whole stay. Check gov.uk for the current fee.