U.S. citizens do not require a Diplomatic Visa to enter 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 Britain.
Your passport must have adequate unused visa pages to allow for entry and exit stamps upon arrival and departure from 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.