If a passenger gets on a train without a ticket or promise to pay notice at a station where ticket buying facilities are available, they may be liable to pay a Penalty Fare. The penalty is the greater of £20 or twice the full single from the station where they got on the train to the next station at which the train stops. If the passenger wants to travel beyond the next station, they must also pay the relevant fare from that station to their destination. If you refuse to pay the Penalty Fare, then you may be prosecuted under criminal law. Full details about Penalty Fares can be found here.