There is a very useful book which I can't recommend too highly - edited by Paul Oliver Black Music in Britain, Open University Press, 1990 (and now sadly out of print).

There were black musicians in Britain in the late 18th/early 19th century, such as Billy Waters who was caricatured by Cruikshank and Pierce Egan in Life in London. The later minstrelsy craze included a few black performers. Black sailors contributed to the development of the shanty singing tradition. There were early black performers in jazz after World War One such as Gordon Stretton, and also a number of African singers who recorded in London before World War Two.

The impact of the Windrush and subsequent migrations on Britain and British music has been well documented, but there are a lot of other strands that still need to be researched. A recent exhibition in Liverpool noted the invisibility of the city's black community in the Beat era but their contribution was real and significant, even if it wasn't much noted. See this site for more information about Black Music on Merseyside.

Matthew Edwards