National Rail Conditions of Travel - Part D: Using your Ticket

15.1 Some train services include first class accommodation. Where first class accommodation is available, the relevant seats and area(s) of the train will be clearly marked. Unless Train Company staff, or notices on the train give you specific permission, you cannot travel in first class accommodation (including standing in corridors or passageways) with a standard class Ticket. This applies even if there are no vacant seats in standard class.

15.2 A standard class Ticket can in most cases be upgraded to first class. Where this is the case, you should pay the additional fare due before you board the train. The amount you pay will be the difference between the fare you have already paid and the appropriate first class fare for the journey you wish to make.

15.3 Some Train Companies allow you to upgrade on the train on payment of a supplement. Where this is the case, it will be made clear by announcements or notices.

15.4 If you travel in first class accommodation without a valid first class Ticket or permission from staff to occupy that accommodation, Condition 9 will apply.

15.5 If you are given permission to sit in a first class seat when holding a standard class Ticket, it is on the basis that you may be later required to give up your seat to a passenger holding a valid first class Ticket.

15.6 If you hold a first class Ticket and the first class accommodation on your train service has been declassified or you are unable to sit in it for all or part of your journey because it is full, you will be entitled to a refund as set out in Condition 31.