Just two remarks:

1. Award availability on SAA in business class is often a problem. You will need to be pretty flexible / lucky and start looking well in advance.
2. The UK APD (air passenger duty), is indeed close to extortionate for long haul premium cabins. It is a departure and not a landing duty (and as mentioned is not charged when transferring).

Having said that, the SAA LHR/CPT flight would be very suitable for your outbound travel. Although JNB has improved substantially in the last few years, I still try to avoid it when coming home! The only other *A international arrival from Europe into CPT is the seasonal (SA summer only) LH flight from FRA (seats not true flat bed like SA but 'slanted', still an option though as there is often slightly better availability).