Hauteboy, no. You can only use the BA franchise partners, and not all the codeshares. In fact, to complicate things more, there are flights that are listed under a BA flight number, operated by a BA franchise, but flown on a non-franchise carrier. For example NBO-HRE BA8669. It is a BA number, but operated by AirKenya under Regional Air. Regional Air is a franchise, but Air Kenya isn't, so you can not use this flight.
Does anyone here have any experience on the JNB quagmire?