Your identity document (for example your passport or identity card) will be checked when you arrive at a UK port or airport to make sure you're allowed to come into the country. It should be valid for the whole of your stay. You may also need a visa to come into or travel through the UK, depending on your nationality.
U.S. Passport. Trusted Traveler Program card (NEXUS, SENTRI or FAST) (see below) U.S. Military identification card when traveling on official orders, or. U.S. Merchant Mariner document when traveling in conjunction with official maritime business.
The UK Electronic Travel Authorization is a new visa waiver for non-visa nationals. Although Americans do not need to apply for the ETA yet, they will need to in the coming years.