How do you commute from Johannesburg? It must be both expensive and time consuming? Lots of spare time?
And hopefully you then take sufficient rest before commencing your period of duty.
Perhaps BA should insist that all commuters have at least as much local rest near LHR (or LGW) as BASSA or BALPA would require them to have after an on-duty positioning flight of the same duration. Monitoring might be difficult and considered intrusive, but detecting use of staff travel flights in the "rest" period should be possible. And BASSA could hardly put up an acceptable argument against the principle.