Sparks wrote:

Not sure, but at a guess I would say, when they stopped using fuses and used
MCB's!

Long before that. Circa 1947 would be my guess, when BS 1363 and the
ring circuit first came along. A consumer unit is the combination of a
main switch (or main switch-fuse, as they usually were, pre-war) and a
fuse box. Double-pole fusing was also the norm before WW2, leading to
all kinds of dangers.

--
Andy