I think it must have been to do with the local exchanges.
Interesting logic re: disconnections. I don't know the answer to that one.
My parents number in Queen's Park was six digits from the start (about 1968). But I also saw a number around the time less than half a mile away that was five...
It's surprisingly wrong. Woodingdean/Rottingdean were still five numbers in the mid 80s as was the centre of town. Being a pedant, I've just looked at the 1983 phone book in Ancestry.
I think I can help, well, a little.
There was no set date. It was phased.
In the area I grew up in, Queen's Park, six digit numbers existed from before 1960 (but some stayed at five digits after then). However, in Patcham there are 5 digit numbers going to later in that decade and possibly...