I remember hearing somewhere that it was 111 but interference on overhead phonelines during bad weater could cause this number to be 'dialled', so it was changed to nine because analogue phones had to pulse 9 times to send a nine and itwas less likely to be cause by interference.
Before there was dialling, it was different, of course. In a public phone box there was a big white button on the wall above the phone marked 'Emergency'. This was because you only got connected to the operator after you had put your 4d in. For the few who had phones at home, you lifted the receiver, waited for the operator, and asked to be put you through to the emergency services.