Visitors from the United States, for the most part, do not need Advance Entry Clearance, also called a Visa.
If you have previously been refused a Visa or if you have previously been refused entry into the United Kingdom, you will need to apply for a Visit Visa before you wish to travel to the UK.
Official FAQ from the United Kingdom Border Agency on Visitors: www.ukba.homeoffice.gov.uk/visitingtheuk/visitors/.
Cost: Currently the cost for a 6 month visitor visa (single entry or multiple entry) is £67 or $114 + $12 return postage fee.
Timeframe for processing: It is always a good idea to apply for your visa BEFORE you make travel plans as it is not guaranteed that your application can be processed in a short amount of time.
How to apply: Go to the UK Border Agency online application website. This can be found at www.visa4uk.fco.gov.uk/ and click ‘Apply for a Visa’.
As always if you have further questions we invite you to visit the UK-Yankee Online Forum and post in our Visas & Citizenship section and the members there will do their best to assist you.
You can also seek professional advice & assistance. You can find a UK approved immigration advisor at: www.oisc.gov.uk/people_seeking_immigration_advice/.
Please note that this information is current as of 17 August 2009.