Towards the end of the 18th Century the burial registers of Deptford St Nicholas contain a number of entries where the deceased is decribed as 'A Lascar from a Ship'. Lascars, in this context, were Indian sailors employed on British Ships.
Perusing the Baptism registers for the beginning of the 18th Century, I came across this entry dated 28 January 1704 for Anthony Black:
Anth. Black being an Indian born at Maddaras bapt in ?? Church
Anthony Black is unlikely to have been the first non-white in Deptford, but he the earliest that I have found recorded, so far.