Contact the American Embassy in London on the phone numbers 0870 025 0249 (at 13p/min + network access charge) and 020 7499 9000 (at UK standard rates) for visa applications, asylum enquiries and for support if you have lost your US passport whilst in the UK.
To contact the American Embassy, call their switchboard on 020 7499 9000. If you are specifically looking to organise a visa appointment by phone, call their UK number 020 3608 6998.
The American Embassy to the UK is located on Grosvenor Square in Mayfair, London. It is the first place to visit for American citizens in the UK as well as travellers or prospective migrants looking for visas.
The American Embassy in London is the largest one in Western Europe. Its website provides comprehensive details on obtaining visas for both immigrant and non-immigrant travel.