Have you a specific journey in mind here? Might be easier to answer the question with more information.
I am also fairly sure there are some cases where you can't know, without having local information.
FRPP said:Over-riding - Travelling beyond the destination on the ticket
A customer travelling beyond the destination on the ticket should charged a full single or return for the additional journey. If it is cheaper, the difference in fare between the fare already paid and the price of the full single or return fare for the journey from their originating station to their destination station may be charged.
You can't, but you could be (over distance) excessed. An excess makes the old ticket become whatever the new ticket is, so there is of course no requirement for the train to call at the destination of the original ticket. I'm not saying that is what they would/should do, and in most cases common sense would hopefully be applied.I'm pretty sure the penalty for contravening this would always be that issued for travelling off-route. After all, how can you be sold an extra ticket from a station the train didn't stop at?