I do not, in general, think of the Romans as the settlers of Britain, though some obviously did, not the way the Anglo-Saxons were settlers, but as primarily the military occupiers.

I think that's right, and I suppose that's one way in which the parallel between the Belgian Congo and Roman Britain is more exact than that between the Belgian Congo and the Eastern seaboard of America, whether of the sixteenth or seventeenth century. Perhaps if he'd relocated Marlow to California he might have found a better match in the Spanish colonists?