I have flown UK-SA 7 times since early 2012, I paid £680 that time, and the fare has been in the £620-£900 range ever since.
I mostly fly KLM, but have also flown Lufthansa, SAA and Virgin too
Bottom I would put Virgin...terrible service and uncomfortable seats..
Then Lufthansa, quite good cabin service, but seats as hard as rock!
SAA are fairly good, if only the IFE worked!
KLM win for me by a country mile...great service, great IFE, and a nice comfy seat..
My mother in law prefers Emirates when travelling to SA, but says that EgyptAir are very cheap...not sure that last one is a good option now!
BA I wouldn't touch with a bargepole either!
Keep looking at the fares, they go up and down like a fiddler's elbow!
But with oil prices staying low, you may yet get a bargain...