for most brits , their first encounter with people of colour came with the arrival of GIs

Eh?

Black (and Arabic) persons have been around in these islands since the Middle Ages, first left over from the Roman occupation and then as a result of seafaring. The term of Tudor times was "blackamoor", not intended, I don't think, to be very complimentary. On Tyneside the very long-established Bengali community is dubbed (by some) the "South Shields Arabs". Bristol, Liverpool and Cardiff also have very long-standing black communities with nothing whatsoever to do with WW2 GIs and predating also Enoch Powell's recruitment of Afro-Caribbeans for the NHS.