In fact all those involved with transporting agents across the line were considered a spy and were to be shot, that included the aircrew transporting them even if in uniform.
Captain Triggers meets Mdm Boissier who identifies herself to one and all as a spy. This is incorrect according to W.E. Johns (RFC pilot WWI). Contact with those requiring transportation across the line was done quickly, quietly and with as few in on it as possible. The fewer who know the more secure the agent would be as there were German agents around all aerodromes to observe and report.
Lt. Bravington, a newly arrived replacement pilot, takes it upon himself to attempt a rescue of a fellow airman downed over enemy lines. All new pilots were introduced into combat over several weeks during which they learned the routine of the unit, local geography & navigational landmarks, and aerial combat tactics. It is therefore highly unlikely that he would be permitted to take a valuable aircraft on such an operation as green as he was. (Ref J. McCudden RFC "Flying Fury")