Are you sure? I know it was used freely among the Brits. One of Ian Fleming’s Bond stories had a chapter entitled “Nigger Heaven.”
The chapter was retitled in the American edition.
The Brits used it for any dark-skinned people, like darker Indians, not necessarily just Africans.
The brits got it from US southerners and some of the British dialects have the same pronunciation peculiarities. They did not use the term Negro much to start with. That was a Spanish word that came in to the US through New Orleans and Florida.