Sorry for the late reply on this one. My wife and I did this last year and are planning another trip for next June as well. The best way to get a deal on a premium fare without killing your wallet is to book the ticket round trip from London to JNB and then buy a cheap BA or SA ticket to CPT. You will also of course have to purchase an upgradeable, or mileage ticket from the US to London, but those are usually pretty easy to come by. As an example, we were able to book tickets last year RT London on Iberia, connecting in Madrid, in Biz class for $2,200 per person. Next summer looks like we'll stick with Skyteam and fly AF, connecting at CDG for about $2,800 per person.