While Britain participated in the slave trade, they went out of their way to not do so in Britain; the slaveholders kept their slaves abroad and out of sight, in places where the laws of civilisation which rule such institutions to be obscene are waived. (Sugarcane plantations in the Caribbean were one such place.) And it was impolite to inquire too hard about the sources of fortunes that built stately homes.

The difference being that when the N-word was uttered in Britain, it was belittling and condescending in an exoticising way, like a colonial-era picture book of funny foreigners in their traditional costumes. In America, however, it had a more immediate edge of aggression and violence, implying an other that’s here rather than far away, and firmly below the speaker in the established racial hierarchy, and who must be kept there using tools including designating them as a (N-word).