Next month I'll be travelling to South Africa, and I need to figure out how long to plan for a transfer through JNB. I'll be arriving on a domestic flight from either Port Elizabeth or Cape Town, and then transferring to BA flight 56 to LHR.

I can't tell from any of the online resources whether or not it is possible to move airside from the domestic terminals at JNB to the international departures area. Can anyone who has connected between the domestic and international BA operations in JNB provide some insight? Does it make a difference if I fly SA domestically and then transfer to BA?