I am planning to travel Carlisle to Cambridge via London, but want to take time to follow Cumbrian coast. Trainsplit suggest a VT ticket which includes Lancaster to London with connecting services. Is it reasonable to expect a Northern guard to accept my method of connecting Carlisle to Lancaster?
Virgin Trains & Connections tickets are valid on appropriate connecting trains.
Providing you follow the itinerary shown on your booking confirmation, you should have no issues.
If train operators don't want people travelling on their trains for connecting purposes they can specify this in the data.
So if you obtain an itinerary for a particular route / service, the ticket is valid on that service and the itknery is evidence of the contract entitling you to take those trains.