Welcome to FT!
(1) You could ride business class which would not be all that bad for 120K each.
(2) Use UA miles to upgrade a purchased fare to Europe, then use miles (75K in business, 100K in FC) to South Africa from there.
(3) Ride MaxJet's new all-business IAD-STN RT for $1K and then use UA miles from LHR.
The Europe stopovers are easier on the system, too.