There are two U.S. Missions in the United Kingdom that process visa applications:

the USA Embassy London
&
the US Consulate Belfast

US immigration London

The US Embassy London is based at Grosvenor Square in Central London. The American Embassy London is the busiest US Mission in the United Kingdom and deals with thousands of visa applications a year for persons seeking a UK American Visa (i.e. US visa issued in the UK).

For all temporary visa applications a US Embassy London DS156 form must be prepared and submitted along with the relevant supporting documents and forms.

The US embassy has been in Grosvenor Square since 1938 and moved into the nine-storey building at the current location in 1960.

The U.S. Embassy London is unique in terms of the US diplomatic set-up as a result of the lease arrangement with the Duke of Westminster – virtually every other major mission is owned outright by the US.

In October 2008 US Ambassador to the U.K., Robert Tuttle announced that America is to build a new embassy in Wandsworth, south London

The US Embassy London also has a specific E treaty visa unit for the processing of all E-1 treaty trader and E-2 treaty investor applications of persons resident in the United Kingdom.

The US Consulate Belfast is based in central Belfast at Danesfort House, within close proximity to the famous Danesfort Park.

There is also a consulate In Edinburgh and an American Embassy Welsh Affairs Office. However they do not process visa applications.

There is also the US Embassy Dublin for applicants who reside in the Republic of Ireland.

US Embassy London: http://london.usembassy.gov/
US Consulate General Belfast: http://london.usembassy.gov/nireland/
US Embassy Dublin : http://dublin.usembassy.gov/
US Consulate General Edinburgh : http://london.usembassy.gov/scotland/
US Embassy Welsh Affairs Office: http://london.usembassy.gov/wales/index.html