Andrew Gabriel wrote:

They were all killed off sometime around 70/71 when it became illegal
to sell electrical fittings which didn't conform to a relevant
British Standard.

Slightly later than that I think. The big change was the coming of
"Council Directive of 19 February 1973 on the harmonization of the laws
of Member States relating to electrical equipment designed for use
within certain voltage limits (73/23/EEC)" - generally known as the
low-voltage directive (LVD). The UK implementation was the Electrical
Equipment Safety Regulations of 1973, subsequently revised in 1975, 1989
and 1994.

--
Andy