Just read this in an article and never knew it existed. I hope that no one reading this is ever in this situation but if they are it could be a life saver.
If you are unable, or it is unsafe for you to talk when you call the police, there is a procedure to follow called Silent Solutions.
If you dial '999' (or '112') and don't reply, the assistant will ask you to 'cough', or make some other noise in the case of a police emergency.
If making any sound is dangerous or you are unable to make a sound because of any other reason your call will be put through to an automated system which will ask you to press '55' if your'e in trouble.
When this happens, the system transfers the caller back to the original operator and further attempts to establish communication or trace the call made and hopefully gets someone out to you as quickly as possible!