If your flight is on time and you have a following wind.

Normally BA get the end gates of the new International Terminal extension, and it is a long treck to to imigration, similar to the walk from the end of LHR T1 to imigration. Baggage is usually off by the time you get through imigration.

A short walk and up a couple of levels of esculators, and your at Domestic departures where if you have bags you'll need to drop them off, and for some bizaar reason they will give you a new boarding card.

Often there are queues to get through security, but they seem more relaxed in SA about gate closing times.