Emergency SMS is a free service that allows you to use SMS (text messaging) to contact any of the UK 999 services (police, ambulance, fire & rescue or coastguard).
It is available for anyone who finds making a voice call difficult, has communication difficulties, is deaf and includes members of the autistic community. To use the service, you need to pre-register your phone on the website.
Please see more at: http://www.emergencysms.org.uk/
This may be useful to a great many people, including those who may want to make a 999 call without being seen to do so.