Hi,
I dont work for BA but I work for another airline as ground crew.
When we check in we can display a seat map however it only shows a very basic map showing whats seat are taken, what can be assigned and what seats are over the wing. We know what seat are "window" seats because of the letter relating to that seat eg: A and K windows and sometimes L but it depends on the config of the aircraft. Unfortunately it doesnt show if a window seat doesnt actually have a window.
As I know how frustrating it can be getting a wall when you really want a window I have checked on our aircraft what ones do and dont and I leave them to last. If i do have to give a wall window out i tell the passenger.
But i guess check in agents for airlines that operate different types of aircraft and configs it would be hard for them to know them all.
Well I hope in future you get your window seat with a window.
happy flying