TICKET OFFICES
Ticket offices must sell the cheapest through ticket valid for your journey, to the best of their knowledge, unless you request a specific ticket.
Ticket offices can refuse to sell you a ticket (or tickets) which is not valid for your journey, if they believe you will use it when it is not valid.
Ticket offices that do sell you a ticket that is not valid must inform you of when or why it is not valid and stamp it 'validity advised' or similar.
Ticket offices must tell you when or if the ticket is not valid.
TICKETS
Tickets now carry the date, time and location of issue. The guards can see where you bought your ticket and can judge that you bought, for example, your Gretna-London ticket from Piccadilly 10mins before you boarded the train there.
I have not checked yet, but I would imagine the Gretna-London ticket, when used as such, would be valid via Manchester. The route being Gretna-Preston-Manchester-London. However, you could only change trains at Preston and/or Manchester (or anywhere else along the route) and use station facilities but not actually break your journey, unless you were unable to complete your journey on the day before.
There was mention earlier of the use of two tickets on some journeys, for this purpose I add:
Where two or more tickets are used for a through journey, the tickets must, have a common changeover point where the train used stops, be zonal tickets, or be one season ticket (which for these purpose do not include tickets issued by or on behalf of local authorities) and one not. Just because a guard fails to notice this, it does not make it valid.
For example:
If you had a season ticket from London to Milton Keynes, a return from Milton Keynes to Carlisle, and a return from Carlisle to Glasgow, you could board a direct service to Glasgow provided it stopped at Carlisle. It would not need to stop at Milton Keynes as the first ticket is a season and the second is not.
If the Milton Keynes to Carlisle ticket was a season ticket you could take the direct train provided it stopped at Milton Keynes, as the first and second tickets are season tickets. It would not have to stop at Carlisle as the third ticket is not a season.
If the first and third tickets were Season tickets but the Milton Keynes to Carlisle ticket was not, the train would not have to stop at either changeover point.