Performance improves the colder the temperature. Hot air is thinner so you need more through the engines and over the wings.
I think you may not realise quite how big Africa is, heres the distances by great circles of the flights you mention
LGW-CUN 4313
LGW-MLE 4597
LGW-OAK 4676
LGW-JNB 4862
MAN-CPT 5336
Its actually further than UK-Phuket which i believe was done with a stop.
It is much further than any of those routes, I guess you'd need around 14T more fuel than CUN thats 140pax and bags. Also There are no nearby alternates to CPT so you need Bloemfontein/Joburg or Durban diversion fuel which again is another 7 or 8T fuel over any of the others. Thats a lot of payload.