Your itinerary is six continents (Europe/Middle East-Africa-Sth Am-Nth Am-South West Pacific-Asia)

For Business Class, the base fare is:
- ex-UK GBP7900
- ex-SA ZAR80500 (about GBP3700)

So there is a substantial saving if you start in South Africa
(note to the base fare must be added the various taxes, fees and surcharges - but these will be about the same no matter where you start)
Remember that you would have to get yourself to South Africa at the start; and the rules will require you to finish your RTW back in Africa*

Yes Africa to South America is possible:

• the Brazilian airline TAM is planning to commence Sao Paolo to Jo'burg flights sometime this year
• even if you start in the UK, the rules allow you to return to Europe/Middle East from Africa on your way to another continent such as Sth America; you are only allowed to transit Europe/Middle East, meaning no longer than 24hrs. And you cannot transit through your country of origin (in other words the UK). Iberia is recommencing JNB-MAD flights mid-year, so you could fly JNB-MAD-Sth America. Qatar Airways is another possibility - it flies form CPT and JNB to DOH and from there to Sao Paolo and Buenos Aires
• if you start in South Africa then there is no issue travelling via Europe/Middle East to Sth America

(* the fare rules define Nth Africa to be part of Europe/Middle East, so nowhere there can be your end point)

Feel free to post your proposed itinerary here - plenty of us are willing to make suggestions