If you board a train without a ticket valid for your entire journey, you may be liable to pay a Penalty Fare of £100, plus the price of the appropriate single fare for your intended journey. However, if it is paid within 21 days, the Penalty Fare is reduced to £50 plus the price of the single fare.
The Penalty Fare will provide authority to travel to your intended destination on that train. If you need to change trains during your journey, you will have to purchase an additional ticket before boarding the next train.