I have no idea how many JNBs BA currently do (hence my words!) they ALSO run to CPT every night.
AFAIK, the run to JNB a standard x7 with two days of an extra (x9) and they sometimes run a daylight on one day a week (x10) but I did not bother to count.
The example shows that, if JNB can support at least 25 rotations a week from LHR - there is enormous demand. A friend currently looking to go out in September has been checking the dog-leg routes of LH, KLM, Emirates and the rest. LH do a competitive all-in including the feeder from the UK.
I have used several of the dog-leg routings myself over the years. The demand is astounding and the reasons are fairly obvious.