10:40 should be possible at JNB, but you'll be on different PNRs so if anything goes wrong with the LHR-JNB, you'll miss the CPT flight (and miss out).

AFAIK, you can route a redemption with a stopover at JNB for the same miles(especially if CPT direct isn't available - i've done it myself) but obviously, the JNB-CPT leg will be in J, not F. Might be worth trying to see if you can change it - that way, you'll be on the same PNR and won't have any problems.

Oh, and I don't think BA will allow you to check your bag through - you'll need to clear customs at JNB and if you don't have the MCT there's no chance.