US Embassy London
24 Grosvenor Square
London, W1A 1AE
Switch board:  (0)20 7499-9000
Operator Assisted Visa Information
For Operator Assisted Visa Information call 09042-450100 (this is NOT a London number. It does not require any other prefix or dialling code.)
More information: http://www.usembassy.org.uk/cons_new/visa/operator_assistance.html
Service available Monday through Friday, 8.00 a.m. until 9.00 p.m., and on Saturdays from 09.00 a.m. until 4.00 p.m.; £1.20/min; only available in the U.K., within U.K. may be unavailable from some business telephones and mobile networks.
American citizen emergencies
Twenty-four hour assistance is available for genuine emergencies at  (20) 7499 9000.