Hi Chris
The tap is very likely indeed to have a standard thread, no matter
how old (well, at least 1/2 a century). If the plastic connector
goes on without splitting or the threads being chewed up, that's
not the problem.

Dead right. All threaded taps are BSP threads, even in Europe. In 30 years
of connecting pressure washers to taps I've never found a domestic tap that
wasn't 3/4" BSP.

In industry you occasionally find 1" and rarely 1/2", but as you say, they
are chalk & cheese.

It almost has to be 3/4" BSP which means the adaptor may be faulty - bad
moulding or something. Try another one!

Dave