My understanding is that the case law from London & North Western Railway Co. vs Hinchcliffe refers to a purchase of a ticket which did not permit BoJ, whereas modern season tickets, and most, but not all, flexible tickets, do expressly and unequivocally permit BoJ, as confirmed by ATOC and the DfT.
Rail franchises provide that tickets for rail travel are sold, and the contract for travel formed, subject to the NRCoC, and Conditions 16 and, in the case of a season ticket, 30, do clearly permit starting and finishing "short" at internemediate stations.
I am not legally qualified, but have studied contract law as part of my studies towards a professional qualification, and am firmly of the opinion that a passenger who purchases a season ticket has entered into a contract for travel which expressly permits starting and finishing "short" at intermediate stations.
This is in addition to, not instead of, the right to travel from and to the stations specified on the ticket.
However, in the absence of this specific right, this would not be permitted, since the ticket would then only permit travel from the origin station stated on the ticket to the destination station stated on the ticket.
If the conditions of an A to C ticket do expressly permit travel from B to C (as would be the case here), then this travel is permitted under the contract for travel, and the passenger has paid a correct fare for the B to C journey, regardless of whether the B to C fare is higher or lower than the A to C ticket.
Since the passenger has purchased the A to C ticket, he or she has entered into a contract for travel from A to C except insofar as the conditions of the contract give the passenger the additional right to make journeys to and from intermediate stations.
The passenger has not purchased the B to C ticket and so the conditions and pricing of the B to C ticket is of no relevance to him or her. Accordingly,
an A to C SVR ticket with restriction code 2T (which permits BoJ) could be used to travel from B to C even at a time when a B to C SVR ticket with restriction code 2R forbids travel.
A further analogy would be with a passenger who travels using a combination of tickets, meeting the conditions of NRCoC 19, in order to save money on the 'through fare'.
Case law on 'tax avoidance', such as:
Fisher’s Executors v CIR [1926] AC395
Duke of Westminster v CIR [1936] AC1
Ayrshire Pullman v CIR [1929] 14TC754
provides that a person is entitled to minimise tax paid by lawful means, and I suspect that the distinction between 'tax avoidance' and 'tax evasion' would be held to be equally applicable to the distinction between 'fare avoidance' and 'fare evasion', with the former being legitimate, and the latter being fraudulent.
"Starting short" on a ticket which does permit BoJ in order to save money is akin to putting savings into a cash ISA (subject to applicable limits) in order to avoid income tax, whilst, by contrast, "starting short" on a ticket which does not permit BoJ in order to save money is akin to putting savings into a Liechtenstein-based account and not declaring it.
Since putting money in an ISA, and "starting short" on a ticket which does permit BoJ are both permitted, it follows that the intent of the passenger in saving money is a legitimate one and not fraudulent in any way.
I don't have a link immediately available, but I dimly recall reading a news article from 1997 in which a TOC attempted to PF season ticket holders "starting short" at station B with an A to C season ticket in order to save money on the B to C season ticket, and either cancelled the PFs, or else failed with their prosecutions (although decisions of magistrates' courts do not set case law, they might be considered persuasive). More recently, there have been at least two failed prosecutions of passengers using valid NRCoC 19 splits to save money.
In short, my view is that:
1) it is permitted, given that the conditions of season tickets clearly permit BoJ, and those who suggest otherwise are most likely mistaken;
2) a passenger is relatively unlikely to run into any practical difficulties when making a BoJ at an intermediate station on a season ticket, unless the intermediate station in question is along a line of route which was not intended to be valid but which is nevertheless technically valid according to the NRCoC and the Routeing Guide;
3) if they did encounter resistance, the TOC will probably back down in the end, or else lose in court; it would be absurd for a TOC to successfully prosecute a passenger for doing something, namely, making a BoJ, which the contract for travel between the passenger and TOC expressly permits them to do.
All the above assumes that the journey is genuinely permitted under the NRCoC, and, where applicable, the Routeing Guide. If an online journey planner mistakenly showed an A to C journey as being valid via a false intermediate station B, then the situation is complicated somewhat, and a passenger who knowingly tried to take advantage of the journey planner's error in order to make a journey which is not actually permitted under the ticket conditions would likely be committing a fraud.
If a passenger intentionally misrepresented their intended journey at a ticket office in order to obtain a cheaper A to C ticket genuinely valid via B in order to make a (valid) BoJ at B, then this could well be a fraudulent misrepresentation, even if the ticket so purchased is actually valid for the journey made.