Technically it is. If we use the old definition of BoJ, i.e. leaving the station premises, then how is it possible to use a Lancaster - Euston ticket, start at Preston and not leave the railway's premises between Lancaster and Euston?
What old definition? The CoT are a 36-page standalone document. There is no requirement to refer to 'old definitions'.
In full:
"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.
16.3 Where a break of journey is allowed, there is no limit to the number of times that you can do so within a Ticket’s period of validity, until the journey is completed.
16.4 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. However, this may not be the case with some through services that take an indirect route. You may also end your journey (in either direction in the case of a return Ticket) before the destination shown on the Ticket.
INFORMATION: For example, where a train service makes a circular journey, you may travel either way to the destination on your ticket. However, you would not normally be allowed to get off at an intermediate station where the fare would have been higher.
16.5 If you start, break or resume your journey at an intermediate station where you are not entitled to do so, you will be liable to pay an excess fare. The price for this will be the difference between the amount paid for the Ticket you hold and the lowest price Ticket available for immediate travel that would have entitled you to start, break or resume your journey at the station concerned.
"
Nowhere is starting short defined as BoJ. It only says that indirect services may have exceptions.
Nothing about buying a ticket from Glasgow but getting on at Preston.
A break of journey is a break of journey. Starting short is not by reasonable meaning 'break of journey'.
Stopping short is more reasonably construed as BoJ, as conceptually you could resume. But when you start on a Lancaster ticket at Preston, that's the start of the journey. How can there be a break if the journey only just started?