If you are caught on the railways without a valid ticket, you have the chance to pay an on-the-spot fine, or at least part of the fine there and then. If you cannot do this, it effectively means you never had any intention of buying a ticket in the first place, and this is a serious offence nowadays which carries the penalty of criminal prosecution (theft and trespassing to name but two offences).
It will cost you hundreds of pounds in penalty fines and court fees, plus you will get a criminal record which stays on your file for at least 5 years (all criminal records apart from child-sex offences are wiped off within 20 years) and it will show up whenever somebody does a criminal records check on you.
It won't affect education, but it probably will affect some employment. In some countries (including USA), it is impossible to get an entry Visa if you have a criminal record.