This was in a mailing list but it was worthwhile copying into here:

Just to clarify how the UK emergencySMS service works.

You do need to register your mobile phone number by sending the word ‘register’ in an SMS text message to 999 and then reply to the message you get back. This generally takes less than a minute so you could do it in an emergency but it is better done before.

Then if you have an emergency write an SMS message with the emergency service you want, what the emergency is, and where you are e.g. ‘police robbery at post office 123 the high street kingston’ and send the SMS message to 999. We will then connect you to the nearest emergency call centre and they will send SMS messages asking you questions about the emergency.

While some UK Emergency Services have local SMS numbers the emergencySMS service uses 999 to provide a service to the whole of the UK that will connect you to Ambulance, Coast Guard, Fire Rescue, and Police.

I hope this helps explain the emergencySMS service and if you have not registered your mobile phone please do it now.

With thanks to Michael J Day of BT for this information

Further information on http://emergencySMS.org.uk