Chose Scotrail & Chiltern Railways because they were home to Classic train simulator 22 Turbostar route. Northern Rail it's my third choice because Turbostars Class 168,170,171,172 can couple to a Class 150 Sprinter something that they might actually do on Manchester Victoria to Leeds City via Halifax and Bradford Calder Valley Line.
All of those can couple to the 15x sprinters bar the 171 which can couple to the 377 and would be good for ECS on BML between Wivelsfield and Selhurst
I recently suggested the North Warwickshire lines using the Class 168 and 172, which would be an interesting route for these trains. (My suggestion seemingly got buried behind pages of other suggestions within several days, and I don't think many saw it )
And to be honest, I’d like that too because it’s a modern diesel route with Turbostars and Chiltern Railways