Do you really mean BS4343? They're the heavy-duty commercial types in various pin configurations and color coded for different voltages. I'm not sure when they were introduced without looking it up.
The 13A plug bottom left of the pic is to BS1363. It was introduced with the ring circuit in the late 1940s, I believe either 1948 or 1949. The older round-pin styles continued in use for many years though.
