the relevant Train Companies will make this clear in their notices and other publications
The problem is that they don't really make it clear!
For instance, FGW say that you cannot 'break and resume' a journey (on an advance ticket), but without mentioning what happens if you don't intend to resume your journey.
As I recall, if you buy a ticket from thetrainline, it doesn't say anywhere that you can't start or finish short, even on an advance ticket. (just that you must travel on the booked trains)
NXEC are clear about it, but only with advance tickets.
The above reads as stating that the rights of starting, breaking and resuming a journey at an intermediate station may not apply if break of journey itself is prohibited.
While I wouldn't disagree with that - it's certainly a reasonable interpretation - I don't think that the train companies, in general, have actually make it clear, when they could have done very easily.
It's quite reasonable for somebody to assume that you can terminate your journey short without paying extra - that's just common sense.
Because of that, I think that if travelling short is not allowed then they should really say so explicitly, but I guess that they don't think it is important enough to waste 5 minutes on!