"That isn't to say there were no fiddle traditions in Africa at the time in question, because there were, but they were almost exclusively North African in origin, like imzad fiddle music. It is my understanding that there just weren't many North Africans living in Britain at that time (1700s), and the few of them there were, certainly didn't have the power or influence Greg is suggesting upon the English fiddle tradition, as their music tradition roots were from sub-Saharan Africa, and their presence in Britain being due to their participation in the British slave trade." (One ofvthose GUESTs without names attached)

Of course there'd have been a fair number of people from the British Isles in North Africa as unwilling "participants" in the North African slave trade. Our history is a lot more complicated and intertwined than is always recognised. It'd be interesting to know if such slaves had any influence on music out there.
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Here is an interesting page about The Black Communities of London which makes a point that after the American War of Independence there was a considerable influx of Black   people from the former colonies, who had backed the Loyalist cause.