The South African routes do involve long downtime at either end for the 'foreign' carrier. But this makes commercial sense because the business demand and therefore yield is for night flights both ways.

I'm not aware that BA does aircraft washing at Cairo. But at least its schedule means fairly short aircraft turnarounds at the Heathrow end - a 747 which has arrived from LAX in the early afternoon can be turned around for an early evening CAI departure, and a CAI arrival at midday or so can be turned around for an afternoon JFK departure (for example).