When travelling with my Mum who uses a wheelchair, we always ask for the wheelchair to be picked up and returned to the aircraft door. (At LHR T5 it is actually delivered to the 'halfway house' area between the plane and terminal.) This has always worked at Aberdeen, LHR T5, Madrid, Granada and Malaga.

(When using the Ambulift it always is left in or delivered to the vehicle.)

I don't have any experience outside these airports, but I would imagine that BA would have to specifically say they can't pick up a wheelchair from the aircraft door or deliver it to aircraft at an airport and state why.