I’m not British, so I don’t know the history of this. The article I took my info specifically said:
Until 2003, you could call directory enquiries (to find out the phone number of someone if you knew their name and address) by dialing 192. That system was privatized, and you had to dial 118 NNN, where the NNN was the number assigned to a commercial service provider, the most famous of which became 118 118.
So the joke in the show was basically, “what if we did to emergency services what we did to directory enquiries”.