Britain was perhaps never the "workshop of the world," but her industrial dominance was such in the middle of the nineteenth century that the phrase is legitimate.
At first I thought the writer wants to say that Britain was not "workshop of the world"; but later the writer says that the phrase "workshop of the world" is legitimate?......like, reasonable?
So what does the writer want to say?
***** NOT A TEACHER *****
I think that the sentence is fine and very beautifully put.
I think that it means something like this:
It is true that Britain was never in fact the "workshop of the world,"
but when one considers her industrial dominance around the 1850's
(and the lack of any real competition at that time), it is perfectly understandable
why historians would choose to award that title to the British nation.