Honestly, I have no idea if there's a Black British population in Singapore. However, judging purely from the looks and accent (neither of which are reliable indicators) of my co-panellist and the history of Singapore, it seems a plausible theory.

Mostly, I think Aaronovitch's depiction of Grant was, to my co-panellist's mind, so convincing it must have come from experience. And it sort of does, as Aaronovitch grew up surrounded by a significant Black British population.