"Wouldn't you describe a Unified thread as Imperial?"

Although many of the unified threads are functionally interchangeable
with BSW and BSF, they are one-for-one interchangeable with the old USS
and NF series of threads, using the same 60°Vee thread and the same pitches,
not including the Brit 26 and 30 tpi. Based on that, I'd have to say
as a minimum, "not exactly" in response to your question. Note that the
Whitworth thread is a 55° Vee.

I got to find out about some of the differences working on a 1949
cast iron BSA Gold Star back in my high school days. Great machine!
(in spite of certain Lucas components) Good thing we were working
in a machine shop in those days!
--
George Downs Bartlesville, Oklahoma