Originally Posted by Globaliser

But if you book LHR-JNB-CPT, when you reach JNB you must clear immigration, collect your bags, and clear customs before taking your bags to the domestic check-in and checking them in again there. Through-checking is not possible for this reason, and nothing to do with interlining or oneworld.

And even if your bag is through-tagged in London (which ISTR it ought not to be), IME the Comair check-in agents will rip off the tag and put a new one on anyway.