Mod Note: Posts #1 - #4 originally in this thread.
Because you can't upgrade a route purely to provide diversionary capability, but you can do if there's potential for the development or enhancement of a permanent service. There is no scope for upgrading the S&C or Bentham lines in the conventional railway sense, though they may work well as tourist operations. As you say Hebden Bridge to York via Dewsbury is already reasonably fast and in reasonably good condition, not something that can be said for the route between Hebden Bridge and Preston via Accrington.Since the 2018 resignalling of the Calder Valley Line East of Hebden Bridge, substantial stretches between there and Bradford Interchange now permit 75-80 mph for multiple unit trains. Not sure how that would help with Anglo-Scottish diversions though!