The NRCoT do NOT prohibit starting short.
"Generally, you may start, or break and resume, a journey (in either direction in the case of a return Ticket) at any intermediate station, as long as the Ticket you hold is valid for the trains you want to use. "
Nowhere is starting short defined as BoJ
16.2:
"
Most Tickets allow you to break your journey. This means that you do not
have to make the whole of your journey at the same time or, where allowed, on the same day"
The intention of this, I think, is to define BoJ as incorporating starting short. Whether it's is significantly clear or not is another matter.
What is the rationale of this odd pricing structure? It seems to make no sense. Not that I am complaining of course, being a Lancaster traveller.
In some ways, it's quite simple: the off-peak tickets to Preston are valid for significantly less time than the off-peak tickets to Lancaster. Off-peak tickets from Lancaster are valid about an hour earlier in the morning than from Preston, and have no afternoon peak restrictions, whereas the Preston off-peak tickets are not valid in the evening peak.
The reason for this is the significantly higher demand for travel to/from Preston in comparison to Lancaster.
I think Virgin have cocked-up here. The price difference which is so great makes so little sense to the average traveller that they feel it's OK to ignore (it's a bit like a 50 MPH limit on a wide rural road...), as well as creating an incentive. While I sympathise with the intention, something I'm sure will have to change with these tickets.