Your itinerary has to take you either Eastbound or Westbound around the world
Starting in SA and heading North to Europe means that you can then head either East or West from Europe

If you head West (to the Americas)
Then you can come back to Africa either from SWP (SYD-JNB), or from Asia (HKG-JNB) or from Europe (LHR-xxx, where xxx is an airport in one of the allowed countries listed in rule 4(e) such as DAR; you will have to transit London - no stopover)

If you head East from Europe, then you have to return to Africa from LHR, using the rule 4(e) exception - again only to one of the listed countries and only transitting London
This is because there are no One World direct flights from the Americas to Africa

BUT, if it's maximising miles you're after then:
HKG-LHR-DAR is significantly more miles than HKG-JNB
SYD-LHR-DAR is significantly more miles than SYD-JNB
and say
LAX-LHR-DAR is just a lot of miles