Runway sweeping at LGW
BOAC, all I can add to your query is what I know about 8U operations.
They operate the A330 on the TIP/JNB sector and then use the A320 fleet to feed from / to EU airports. Occasionally, the A330 will also fly the TIP/LGW sector, maybe twice per week, as scheduled. I was 'lucky' enough to be aboard ONG on a return flight from TIP to LGW a few weeks ago.
I can only guess, but I suspect what was meant was that the aircraft had left LGW, en-route to TIP, then went to JNB before it's fatal return to TIP.