I don't they can easily prosecute for something that is explicity allowed in the case of all day tickets, and all anytime (with or without the day) tickets, and also all off peak tickets unless a restriction code prevents it. In other words, with most ticket types it is fine.
On one walk-on ticket type it is fine unless some tiny little condition that is NOT printed on the ticket, NOT displayed on any website, and NOT likely to be given to the customer in any form, and on THAT basis the customer is going to be prosecuted? No chance.
Old Timer & Helvellyn only have a case against people starting short on Advance tickets, and Off Peak tickets where a restriction code prevents breaking journey and on the basis that starting short is included within the definition of breaking journey, and they would have to prove intent and they would also have to prove that the condition was made available to the customer.
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Source?