If no D inventory on BA JNB/CPT-LHR, you have some options: IB JNB-MAD, QF JNB-SYD (travel to Yale the long way), CX JNB-HKG. There are LOTS of alternative routings, and I guarantee that at least one will have D for most dates ... just maybe not the one you want! The high demand dates to/from Europe and South Africa are SOLD OUT on MANY airlines ... I have seen no A inventory on BA for a MONTH at a time, for example (and had to downgrade to D when I wanted to fly).