Pragmatically the baggage allowance is whatever the agent checking you in says that it is. Once bags are tagged it is rare to have an issue later (but not unheard of).

Contractually the baggage allowance is separate for each carrier, so it can easiliy be different on the BA sector than on the QF sector. QFF status gives you an extra baggage allowance on QF, which is often helpful.

In CPT it is usually no problem getting anything checked through, esp. if you tip the agent (the same applies at JNB, fwiw). Much cheaper than the excess baggage fees. It doesn't work in the opposite direction (ex-Oz).