Prosecution for fraud, which is unlikely, though not impossible. as opposed to a bylaw or Regulation of Railways Act charge, would potentially be far more serious in terms of impact. On the basis of your age and the info provided, it is 99.9%+ unlikely that you will end up behind bars. However, fraud cases can also be heard or disposed of at Crown Court and that is a whole new world of stress and angst. A fraud conviction on your record could also negatively affect your path through life, at least for the next few years.
Cooperate, don't seek to pick holes in the investigation and politely suggest that your lesson is learned. Ask if they would consider a settlement. Good luck