The equivalent UK emergency number of 911 is 999. The European standard of 112 will also work. Make sure you know and can relate your location.

You'll be asked if you require police, fire brigade or ambulance, but you can also request the coastguard. This call is free and can also be made via mobile phone.

In some areas a Pegasus system with PIN code, for those with disabilities which make it difficult to talk to emergency services on the phone. This is in use in Nottinghamshire, Lincolnshire, Dyfed Powys, Surrey, London and Sussex.

Dial 101 to contact police in non-emergencies in England, Wales, Scotland and Northern Ireland. See http://www.police.uk/101

Dial 111 for urgent (but not emergency) out-of-hours medical advice from the NHS in England and Scotland, NHS Direct Wales continues to operate via 0845 4647. 

 They will ask you questions to assess your symptoms, then give you the healthcare advice you need or direct you straightaway to the local service that can help you best. That could be A&E, an out-of-hours doctor, an urgent care centre or a walk-in centre, a community nurse, an emergency dentist or a late-opening chemist

If you have difficulties communicating or hearing, you can use the NHS 111 service through a textphone by calling 18001 111.

If you need to reach the American Embassy out-of-hours, the emergency number is 020 7499 9000. From the embassy website:

In an emergency involving an American citizen in the United Kingdom, you can reach us during normal business hours and after hours, seven days a week, via the main Embassy telephone number: (0)20-7499-9000 (if dialing from the U.S.: 011-44-20-7499-9000). Please note that this number is NOT for visa inquiries.