Interesting! I definitely remember these:
Which were so rarely unplugged that as a child I thought they were permanent. However there were loads of different hard-wired circuits used in the UK, with a wide variety of wiring diagrams required. We only standardised our connection in 1981, which took several years to roll out (the reason was that by standardising the connector, the government gave up the monopoly on phone provision and installation. Now you can buy all kinds of shit phones, instead of the one good phone (in several colours) that the GPO/BT used to provide! Now, the phones that the government used to give us are sold for much more than their original value. Ah, progress.
Incidentally, the faceplate of the socket is the point at which responsibility passes to the homeowner. Presumably when the phone was permanently installed and rented to the user, this also fell to the GPO/BT to maintain.