Isn't that $619 fare from LHR-JNB on Egypt Airlines or Kenya Airlines?
When I was doing my search (I'm there from 6/16 to 6/25), those were the only ones under $1000. BA was around $1200 and Luft was $1400. I searched (using SideStep) from almost every major city in Europe that offered service to JNB and/or could connect in another African city, and I found LHR originations were generally the cheapest.
Now, I feel lucky b/c I was able to snag some BA award tix from LHR to JNB.
I will use alternate means to get to LHR from the States. (either award miles or non-rev).