MiB has it spot on.
You DON'T want to change at JNB. Last year I had to do so for reasons that I shall not bore you with and it was no fun. After 10.5 hrs (even in Club) to reclaim bags and walk half a kilometre to the Domestic terminal and then scrummage in to find a place to wait?
We used the Rennies Exec lounge on Priority Pass and that made it bearable. Then you have 2 hours to CPT with more high quality cabin service from SAA or one of the LCCs. Alternatively, 12 hours direct and you do more sleeping in the cozy BA bed? Heck, that is worth a grand of anyone's money!
cavortingcheetah's story does not surprise me. Although I have not travelled on SAA recently ... I have a family member that is FD crew in SAA and so I know the stories! My last SAA Biz LHR~JNB was (
) 1987 when it was just big seats before beds in biz and the cabin service was brilliant. But it's been only VS or BA since then and it's the route I know the best, as I first travelled it 41 years ago.