A JNB-LHR service is usually planned for crew to achieve 3 hours rest each, this incorporates industrial and legal requirements. Legally, 3 hours rest would then allow them to operate up to a 17 hour FDP. However if the crew rest area was unusable then (off the top of my head) given the report time for the flight lies in the most restrictive bracket then this allowable FDP reduces dramatically to 10 hours with no horizontal rest.

The very least amount of rest to enable the flight to operate would be 1.30 hours giving max FDP of 14.30.

Commander can exercise discretion to get it back to base, which lets say bunk area was u/s completely then I assume this would have been the case.

It is not clear what happened on this particular flight, nor are we ever likely to find out unless a crew member provides an explanation. However I will echo what others say, the crew would never actually actively choose to abandon a service (actually more trouble) but they must adhere to FTL regs.

Thanks