Tourists/visitors from the United States usually do not need Advance Entry Clearance, also called a Visitor Visa. Be aware that as a visitor to the UK, it is against a violation of the terms of your visit visa to do any work or look for work.

If you have previously been refused a Visa, if you have previously been refused entry into the United Kingdom, or if you have a criminal record you will need to apply for a Visit Visa before you wish to travel to the UK.