I have a question about making a journey for which you already have a ticket for part of the journey. The situation is travelling from A changing at B to C and I have a ticket already that is valid for A to B. At A the ticket office is closed but there is a TVM. However AFAIK, TVMs only dispense tickets from the station they are at so I cannot buy a ticket from it for B to C. If there is not sufficient time at B to buy a ticket can I still travel B to C and get one on the train or at the destination without getting into problems?
If facilities at your origin station are not sufficient to buy "an appropriate ticket for your entire journey", you should buy a ticket for part of your journey and then pay the difference at the first reasonably practicable opportunity.
If you come across, or are approached by a guard on the first train, this is a reasonably practicable opportunity.
If that doesn't happen and you have time at the interchange point to go and buy a ticket, this is your first reasonably practicable opportunity.
The guard of the second train might be considered the first reasonably practicable opportunity to many if there was not time at the interchange station and you hadn't seen a guard on the first train, but I have seen it argued that the guard on the second train could not be the first reasonably practicable opportunity if there are facilities at the interchange station, regardless of how long you have there.
However, you have already purchased a ticket before arriving at your origin station and some will say this only allows you to go to the point at which the ticket ends and no further. Others will say it is the same as only buying a ticket for part of the journey at the origin station. A further group might argue that, as it is possible to buy a ticket for the entire journey at the origin, you must do this, even though you already have a ticket for part of the way.
Best advice is get it sorted as soon as, but most guards are reasonable if you pass the attitude test.