Helol
The LHR-JNB route has a somewhat simpler set up - if you want to fly direct, there are only three carriers and they all operate daily, some twice daily. Their fares all tend to track each other.
Also, as it is a prime destination the options for flyer miles upgrades etc are highly restricted. They tend to get booked within days of the 360 day advance opening. That is due to families who have to travel in school holidays and can book early or retired folks who can book the low periods. So I find that ALL FFM options are difficult.
If you want to fly indirect you have all the usual suspects: AMS, CDG, FRA, MUC and then the 'dog-leg' ones that take much longer via the Middle East. Currently, these are the cheapest as one particular carrier (lots of red paint) want market share. Friends of mine have used IST for connections too,
Lastly, the Cape Town market is worse as it is THE destination in Africa and only BA run there all year round. VS still have flights in peak but I seem to recall that they are cutting that to reassign aircraft, following the merger with Delta but I sit to be corrected on that.
All in all, the JNB route is sewn up and you would need to watch for 'flash sales' that the carriers have started doing in recent years. These are all down to the factors that OverRun set out so well.