The relevant extracts from National Rail Conditions of Carriage:
B. VALIDITY OF TICKETS
11. The period during which you can use a ticket
The period during which a ticket is valid is printed on the ticket or will be made clear to you when you buy your ticket. If you use a ticket after the expiry of the ticket’s validity, you may be treated as having joined a train without a ticket and Condition 2 or 4 will apply.
If, as a result of a delay to your train, the validity of the ticket you are using expires during your journey, you will still be allowed to complete that journey. However, in these circumstances, you may not break your journey unless your train is so delayed that a break is reasonably required.
C. USE OF TICKETS
16. Starting, breaking or ending a journey at intermediate stations
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. You may also end your journey (in either direction in the case of a return ticket) before the destination shown on the ticket. However, these rights may not apply to some types of tickets for which a break of journey is prohibited, in which case the Ticket Seller must make this clear when you buy your ticket.
If you start, break and resume, or end your journey at an intermediate station when you are not entitled to do so, you will be liable to pay an excess fare. This excess fare will be the difference between the price paid for the ticket you hold and the price of the lowest priced ticket(s) available at a ticket office for immediate travel that would have entitled you to start, break and resume, or end your journey at that station on the service(s) you
have used.
A ticket which entitles you to travel on the London Underground and/or Docklands Light Railway does not entitle you to break and resume your journey at any of the stations on these networks unless it is a Season Ticket or a Travelcard.12 For the purposes of this Condition and Condition 11, you will be treated as breaking your journey if you leave a Train Company’s or Rail Service Company’s stations after you start your journey other than to:
(i) join a train at another station, or
(ii) stay in overnight accommodation when you cannot reasonably complete your journey within one day, or
(iii) follow any instructions given by a member of a Train Company’s or Rail Service Company’s staff.