Interesting that the origin of the code '999' wasn't exactly known!
It was explained that "they couldn't use '1' because of possibility of overhead wires touching in high winds and '111' being 'dialled'. So '9' was chosen. But why 911 wasn't chosen I don't know" was said
'999' was chosen as the number had to able to be dialled without inserting money from kiosks. The operator could already be called by dialling '0' without inserting coins. This was achieved by using a special dial (GPO Dial No 9 or 11 in the early days of A/B coinboxes) which had a little screw on a cam on the rear of the dial which allowed the 'free' call.
By the simple addition of a similar screw in the position of the digit '9' , it allowed a '9' to be dialled without inserting coins (the screw had to be the next digit to the '0'). There was little likelihood of a subscriber being dialled as there were virtually no numbers that consisted purely of 9's and 0's - except for perhaps possibly 'WOOlwich 9000'
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Thus in 1937 the 999 service started in London - some twenty years before the 911 service was introduced across North America!
It took some years for the '999' service to be rolled out across the UK network. The last exchanges to acquire the 999 service were in 1976! The Aberdeen Telephone Area still had 24 exchanges without a 999 service at the end of March 1976 but all had the service a year later.
Some exchanges with two digit numbering (UAX5 - Unit Auto eXchange No 5) got a '99' service as the number had never been allocated to 'subscribers'. It was the number used for testing to see if there was an alarm present on the exchange. Previously these exchanges had been told to dial '01' (for the operator) 'In case of Emergency' .
There was another error given in the programme but can't remember what it was - much watch it again if I get a minute.