When we were at BAe for our CAP509, we regularly flew to Carlisle on training flights. We even stayed for a week to do night flying as it was so much cheaper than our own dear Prestwick. Gill certainly operated a mail flight then, as we had to stay out of its way. They were using a Shed. The airport itself has customs sheds and a couple of check-in desks, so presumably has had commercial traffic at some point in the past. I'm sure there were old posters or timetables on the walls, but it might be my old memory playing tricks on me.
Worth a visit if only for the sausage and bacon rolls and the wonderful service in the bar.