But we never had an instructional airframe as such whilst they were in service and the only one that ever did was the Trident 3 that had its wings cropped and used for towing instruction after they all went out of service
That's the one I was thinking of. I remember boarding a shuttle to Edinburgh in early 1980 and seeing a Trident in BEA colours, it puzzled me for years until I heard about the one for towing practice. So was it kept in BEA colours all along or repainted as BA then returned to the older livery later?