Try "fly dot com" for LON/JNB
This schedule available with Turkish for GBP403 (ish):
LGW JNB 17:10 10:20 16h 10m 1
JNB LGW 18:45 9:05 15h 20m 1
Or with Ethiopian this schedule for GBP416 (ish):
LHR JNB 20:15 13:05 15h 50m 1
JNB LHR 14:15 7:55 18h 40m 1