In article ws.net,
"Doctor Drivel" writes:

When was it universally adopted? I think the 1960s.

1946 supplement to the 11th Edition Wiring Regs was when it was
officially sanctioned, but people had started using ring circuits
beforehand. Installation was universal by 1950, except in towns
which didn't yet have a 200-250V AC mains supply (13A sockets
were not allowed on DC supplies or AC supplies with no neutral).

snip the rest of Drivel's drivel

--
Andrew Gabriel