Kulula are fine. They are part of Comair (South Africa) and have good standards and maintenance. I used them several times on my last trip. They can be a bit OTT in their welcome and chirpy on board announcements but that's no sweat.
As to getting to/from the UK/ZA then the directs are all expensive. The three carriers all run at capacity and that enables them to keep their prices high. It's been that way for years.When VS started on the route they were cheper and then the discovered just how high the demand is (the reasons for that don't matter here) and so they have been able to charge much the same as SA and BA. No collusion, just that their computers know what their competitors computers are charging!!
The last time I looked at the usual European routes of KL/AF and LH (who have always had good synchronised departures from LHR and other UK airports) they were not that much cheaper. A lot will depend on where you are starting from and your profile says GIB but your post indicated you might be going via London? Consider some of the less obvious but closer to GIB, Olympic, Alitalia, TAP, Iberia, Eqyptair or Swiss, as they all go to JNB.
The middle east routes are cheaper (I am told) but look out for the time delays with the dog-leg and some have a four hour lay-over, I hear from friends..