I think the LBA route was, or could easily be adapted as, a closed diagram using a single aircraft. That would have simplified operation on what was presumably a 'damp' lease with Airways Cymru flightdeck but Midland cabin crew. Teeside might have been similar but IIRC by then had fewer daily rotations than LBA so a/c swapping onto other schedules more likley to be part of daily rotation..
Putting it on EDI or GLA would probably have seen it swapping between routes. Even if it had it's own diagram there would be a problem with differing capacity and seat layouts cp the nines (or 737). More so if weather or other operating exigencies meant a/c had to be 'bumped' forward to fulfil a delayed schedule.