You must have a valid ticket of a Permit to Travel for your entire journey before you go, otherwise you could have to pay a Penalty Fare.
This is at least £20 or double the full single fare to the next station stop. Plus the usual single fare to your destination if you continue your journey.
You might also have to pay a Penalty Fare if you:
Can't show an appropriate Railcard for a discounted ticket
Travel in a First Class carriage with a Standard Class ticket
Are travelling on a Child rate ticket but are 16 or over
Travel beyond the destination on your ticket.
Tickets are available for purchase on-board our trains only when a journey commences from a station that does not have ticket purchasing facilities or where ticket buying facilities are out of order.