Just to add a but more to what's already been said.
You are required to possess a ticket for your journy when you board the train, unless facilities to purchase the ticket you want using your chosen payment method are not available at the station where you start your journey. An open ticket office or ticket machine counts as an opportunity to purchase a ticket.
From what you've told us you boarded the train without a ticket and this is a criminal offence. It is not the same as a parking ticket. TfW are entitled to prosecute you in the Magistrates Court if they wanted to.
What will happen next is they will write a threatening letter to you in the next few weeks saying that they have received a report, are considering prosecuting you but asking for your version of events before they decide how to proceed. It is very important that you reply to this letter. I suggest a short, concise reply mentioning the following points:
- That you are sorry for what has happened
- What you have learned from the incident
- That you are keen to settle the matter without the need for court action
- Offer to pay the outstanding fare and the train company's administrative costs in dealing with the matter
TfW are normally prepared to offer an out of court settlement to people who co-operate with them and who haven't come to their attention before. Expect to have to pay the cost of the fare avoided plus an admin fee, typically £150. Paying an out of court settlement might feel like paying a fine but technically it isn't - only a court can impose a fine as a punishment upon conviction so you really don't want to pay a fine! You won't have a criminal conviction if you pay an out of court settlement.
When it arrives, post a copy of the letter from TfW in this thread along with your draft reply and forum members will proof read it for you. Do redact any personal details before uploading.