Having worked for the world’s favorite airline a few years back, I use to see flights to JNB/CPT/DUR heavily overbooked.
I once saw an overbooking of 100 seats in world traveler. SA is a popular destination from London from a while back.
Even BA staff use to buy firm hotline staff tickets (300-400GBP) to avoid being stuck in either direction.
LGW would be the next choice after LHR I think, since BA has some 744 and 777s based there.
I think it is time SA introduced a second long haul carrier focusing on the European market.
For now it remains BA/VS against SAA.