LHR - CCR
SIN - QR lounge may or may not be open during your transit - check the QR website. (QR is my overall favourite QIN lounge, as it is only available to pax in J/F as opposed to OWE in Y). If you are on BA15 depends what your priority is - If showers then I would do QF J lounge, if a quiet place BA F Bar, If trying to get some laksa then QF F lounge (it is very loud and focuses on serving food rather than a "lounge"). BA F Bar also has the advantage of you will be called for the flight, which in SIN is a very tight timewise when you try to do too much in the lounge. That said for showers I would go QR > QF J > QF F > BA
SYD - QF F lounge