Sir George Cayley
I understood BA taxi along Alpha to the 03L hold to await the numbers whilst other traffic by-passes them on Bravo.
If this is the norm why this departure routed along Bravo may be key to understanding the sequence of events.
That would be the norm if their starting point had been the usual "E" (echo) parking stands
That night the aircraft started out from an "A" (Alpha) parking stand, thus the taxi was thru the main apron which exits directly onto taxiway Bravo.