Attention! Effective January 8: All U.S. citizens (including children) traveling to or through the UK for tourism, business or short-term study for 6 months or less will require an Electronic Travel Authorization (ETA). Please see full details on how to apply for your ETA at: https://www.gov.uk/guidance/check-when-you-can-get-an-electronic-travel-authorisation-eta
Check if you can get an electronic travel authorisation (ETA)

List of nationalities that that can apply for an ETA now or will be able to apply later.

GOV.UK