Celia,
First off - welcome to Flyertalk !
I agree with the other poster that BA is the way to go, but award availability LHR-JNB/CPT in the UK winter / SA summer is very tight. Many people on the BA board talk of booking seats 330 days out (max possible) to get the dates they want for premium class awards.
Suggest that you call AA soon to book, and be flexible with you dates.
Anything else you want to know, post in the
BA forum - they'll be keen to tell you all about Club World