Vampy
At BA, the crewing complement depends on sector length, acclimatisation issues, etc.
BA operate LHR-BKK-SYD, LHR-SIN-SYD and LHR-SIN-MEL.
The (slightly) shorter nature of the flight means 2 pilots ex LHR to BKK and 4 ex LHR to SIN. However the BKK-SYD flight is longer and requires 3 crew versus 2 for the flights onwards from SIN.
So, ex LHR 2 pilots do a LHR-SIN-LHR rotation and the other 2 operate LHR-SIN-SYD (or MEL) - SIN-LHR.
The same 3 pilots operate LHR-BKK-SYD-BKK-SIN.
You tend to get 48 hours off each way in SIN and BKK and slightly less in SYD/MEL (about 34 hours).
The Oz trips leave on day one and return early day 9.
I speak now as a shorthauler, but ex 744 until 2001. Historically there have been some longer trips, I operated a 13-day LHR-SIN-MEL-SIN-SYD-BKK-LHR in 2001. They may or may not still operate, I don't know.
HTH