Fare evasion is a criminal offence and you may be prosecuted. If convicted, you will:
If you're prosecuted, you'll receive a notice in the post that your case will be heard in Court.
You should reply to the Court with your plea, financial situation and any mitigation. If you don't reply, your case will be heard without you and this could mean you have to pay a higher fine.
If you've been prosecuted and weren't aware of the proceedings against you, you can make a statutory declaration to get your case reopened in the magistrates' court.