All sorts of possibilities could happen. Including but not limited to:
- Rerouting you onto other flights to BHD
- Upgrading you for all or part of the journey
- Upgrading others to make way for you in Economy.
My guess is QF will issue boarding passes to SYD and advise to collect BA passes there for the onward flights to the UK and NI and have BA deal with the seating issue (and address rerouting options if seats dont open up) as the flight closes.