From the web site:
Welcome to the emergencySMS
We are trialling a service that lets deaf, hard of hearing and speech-impaired people in the UK send an SMS text message to the UK 999 service where it will be passed to the police, ambulance, fire rescue, or coastguard.
Simply by sending an SMS message to 999 you can call for help and the emergency services will be able to reply to you.
If this trial service is a success then the service will be launched during 2010.
You will need to register your mobile phone before using the trial emergencySMS service, click on the Registering your phone link above for more information.