I believe it is gate 25 the gate with the satellite reminiscent of a Greyhound bus terminal, where the BA shorthaul flights are leaving from. This is not too far a walk as gates in T3 go- but any T3 departure requires traversing the dutyfree and general departure areas. I would say 10-15 minutes from the checkin point, assuming no delays in security.

I am nt sure if AY is also using the bus terminal.