There's good documentation on the UK version of this on wikipedia:
https://en.wikipedia.org/wiki/History_of_telephone_numbers_i...Two letters and a number formed the basis for UK numbers within the bigger cities, and later 0 + two letters + number for national dialing codes when they were introduced later.
So, the big cities (when I was a kid, these were 01 London, 021 Birmingham, 031 Edinburgh, 041 Glasgow, 051 Liverpool, 061 Manchester, so again using the letters) had the local part with local exchange 2 letters + number and a 4 digit number within that local exchange.
Smaller towns and cities had the longer codes (0ABx), again using letters - e.g. the old Coventry code was 0CO3, Bolton 0BO4 etc, and 5 (and later 6) digits for the local part.
At one point when I was still quite young, I lived in a very small village with phone number 205, and the public phone box was 200 - I think the numbers got as high as 230! In addition to the full STD codes, the neighbouring phone exchanges had 1 or 2 digit prefixes so you could call them almost like local numbers. Our village was similar, from the main town you used a 2 digit prefix, and our STD code was the main town's STD code plus that prefix. My dad told me that before the national STD codes were introduced, if you wanted to call another town you either had to call the operator and ask them to connect you, or you had to chain all these access codes together to jump from exchange to exchange until you got where you wanted. Apparently that still worked when he told me, but I was too young to know anyone further than the next town, so I never tested it!