Think we've had this discussion (with respect to Avanti throwing in the towel on their services North of Preston, this during times of disruption) numerous times in the recent past. No doubt there'll be some apologist able to 'explain' why it still isn't possible.
It isn't just about who signs the route, you've still got to have the train crew available.
Let's say 90% of Preston-Scotland services are crewed by Scottish crews (just an example, not an accurate figure). Where does Preston depot magic up the extra train crews to work services North of Preston if they're all already booked to work services south of Preston as usual?
It'll all be a pre-arranged contingency plan between the TOCs and Network Rail. Preston - Carlisle retains a service with TPE. It reduces the number of trains that have the potential to become stranded if the line gets blocked or the power goes. It reduces the risk of units and train crew being stuck in the wrong place. It reduces the risk of importing delays into the rest of network and causing disruption to thousands more passengers journeys, nowhere near the affected area.
People can keep throwing around terms like "operational convenience", but having units and crew in the right place to operate near normal services for the majority of your passengers is an operational
necessity and
good customer service for those passengers.
Added to that, like on previous occasions, Avanti are operating additional shuttles/extensions north of Preston as and when weather conditions, unit availability and train crew availability allow.