I've actually done that a few times for work, always in J. Biggest problem if you do it on BA is that as there is no arrivals lounge, you need to book yourself a hotel room to shower and change in, for which you will be charged full price - as a result I'm not sure I'd do anything shorter than a weekend for a leisure trip as the cost is likely to be the same. LHR-JNB also gets very full so more often than not I've had lower deck E/F seats due to booking last minute, which isn't so great for a solo traveler.

One word of advice: if you possibly can, make sure you get the later LHR-JNB flight - the earlier one lands at something like 4am UK time which makes for a pretty tiring day (but has the benefit of getting you there in time for a 9am meeting in Sandton).

A couple of years ago I did HKG-LHR-HKG in a weekend - took the last CX flight HKG-LHR on a Friday and the lunchtime flight back on Sunday (lands at 7am Monday). I was fine for the weekend but was struggling on the Monday - I would avoid that if possible. I've also done LHR-SFO for a long weekend which was rather more enjoyable (take the 285 outbound on Friday am to arrive at lunchtime, spend Friday afternoon/Sat/Sunday in the Bay Area then get the 286 home Sunday evening, land at 2pm on Monday), and I have to say that was very pleasant.