U.S. citizens do not require a Tourist Visa to enter Great Britain. However, a U.S. passport is required:
Your U.S. passport must be valid for a minimum of 6 months from the date you plan to depart Great Britain.
Your passport must have adequate unused visa pages to allow for entry and exit stamps upon arrival and departure from Great Britain. Please note that Amendment pages in the back of your passport are not to be used for entry and exit stamps or visa issuance.