In article ,
"R W" writes:

What's the story behind the switch from 15A to 13A sockets? Surely 15A
would be more useful....?

The design aim was for 3kW, which is where the 13A comes from.
I guess there probably weren't any portable appliances over 3kW
in use. Kettles of that era tended to be lower power than today's
ones. I have come across a 3kW portable convector heater which
could date back to that era, which is rather ironic as you'd be
hard pressed to find one over 2kW nowadays.

--
Andrew Gabriel