and no one else but the GPO were allowed to fit one
Whats changed? Not a lot.
Ah, you must be too young to remember!
But, yes things have changed a lot!
You can plug any corded/cordless landline phone into the socket, and unplug / plug as much as you like as you change phones manufactured and sold by different suppliers, instantly purchased from a multitude of outlets.
In the days I was speaking about the phones, corded of course, were always supplied by the GPO and had to be physically wired into their terminal box, after they alone had installed all the internal wiring and gone up a telegraph pole to connect your line. All that after going on a waiting list and that could be up to a 1 year wait. There was NO competition to the GPO, who, as I understand, were legally the only people authorized to undertake the work. Adding to all that was the most humble people could not afford a telephone, and if you could, your class of people also did not have telephones due to the price.
Now the speed of the process is totally different, lines/numbers, the telephones and contract can be obtained from others
Nowadays we also have mobiles to add to the competition to landlanes!