Did you book this through BAEC with avios? BAEC charges per segment by class, so you would have paid the award rate for DME-LHR in Y and LHR-AUS in F. If J/F award space became available on DME-LHR you would need to call BAEC to change your ticket and pay the additional avios/cash for this.
I should have mentioned that I bought with AA miles, meaning it's not per segment but F pricing for the entire trip.