Amazingly this is allowed in BA. The rules state that flight or cabin crew (off duty) may occupy the flight deck jumpseat with the permission of the Captain. ID cards must be checked but uniforms are optional (I said uniforms not
clothes 
) The crew members are not allowed to leave the flight deck unless for a toilet break and must be travelling "before or after" a duty. A nice and wide definition deliberately written to allow leeway.
As an example a fellow Captain sat with us on my last sector on his way er um "home"...................
