If you read through this thread you'll find most of the answers to the question. BA and VS can make lots of money on routes GB<->ZA. SA lose lots of money. Next question?
To save you time, read specifically :
http://www.pprune.org/african-aviati...ml#post8213248
http://www.pprune.org/african-aviati...ml#post8331321
http://www.pprune.org/african-aviati...ml#post8473538
http://www.pprune.org/african-aviati...ml#post8594819