There were some previous posts that suggested the plane was park at a gate not normally used, hence using B taxiway ( used mainly for domesic terminal). All I know is that we departed from gate A3 which is pretty normal and as I gathered leads directly on A taxiway. We were also 15 mins early and it was very quiet that time of night. When we left the following night with BA56 we used the same gate A3 and proceeded directly on A taxiway. I did not give much attention to our taxi movement the previous night though.
What might the reasons be why they were directed to the B routing "detour"? Won't this be very unfamilliar territory for a Heavy? Might be normal procedure to use B from gate A3 but looking at the charts it looks strange..