I'd say the upgrade chances are more than slim, but certainly there's a nontrivial chance you'll travel in WTP.
Personally I wouldn't pay $800 more for the return in WTP, but then others may disagree. For me it would depend a bit on what I'm planning to do at the other end. Since you seem to be off on vacations it wouldn't matter so much if you can't sleep well on the plane. On the other hand, you are looking at two 10h+ flights...
I would also take a good look at connection times for BA vs AF: usually you'll arrive in Europe before noon and have a long layover before another overnight to JNB. So you may want to get out into town for a few hours: this is usually easier and more quickly done at CDG than at LHR.