Welcome to FT andyandy21

The "complex rules" are available - here

You haven't listed the itinerary that the online tool gave you, but I suspect that for JNB-MAA it routed you via LHR, right?
And in that case, you have used up your allowed entry/exit to Europe!
(except for a special exemption if you are returning to certain Africa countries - Sth Africa is not one of them - see Rule 2(e)3

So to defer Europe until later in the itinerary, you need to leave Sth Africa and head for either Asia (HKG) or Sth West Pacific (SYD)
Note that the QF service JNB-SYD does not have first class
and as Gardyloo pointed out, CX does not always have first class on its JNB-HKG service

Again as Gardyloo pointed out, you have unused segments
So perhaps a couple of side-trips from London while you're there - say LHR-MCT-LHR and LHR-FCO-LHR or perhaps one longer trip - LHR-DME-IKT-DME-LHR might be nice
Note that there are restrictions on intra-Europe flights - see Rule 2(h)

Finally, because you're starting in Africa, you must end in Africa - anywhere in Africa, there is no requirement to return to Sth Africa - see Rule 2(c)