The problem at JNB is that the majority of international flights (which mainly means flights to Europe) leave within a short period in the evening.

The International terminal CAN be very congested, and if you're not familiar with the layout, it may take more than a moment or two to find the right check-in desks.

I imagine a BA/BA connection would assure better baggage connection than SA/BA. If it were me, I'd allow at least an hour, or preferably longer. OK if you have lounge access.

I have only done this on Friday and Saturday evenings. Other days may be less congested.