I thought it makes no difference to use other person's photocard if I am a student. I used it for convenience but not for fare evasion. Clearly I was too native and should pay for my ignorance and I accept that. But they want me to agree that I would pay for any fare that I avoided. But I did not avoid and did not try to avoid. I don't think I am a criminal.
Is there any chance to argue about that?
Technically, byelaw offences aside, by using someone else's photocard you have committed the offence of Fraud by false representation contrary to section 2 of the Fraud Act 2006.
https://www.legislation.gov.uk/ukpga/2006/35/section/2
So it could be a whole lot worse.
Unless you were dealt with by a Police Officer, you are unlikely to have had any sort of formal warning - i.e. a Police Caution - that will show up on a PNC check.
I can't understand how it would be more "convenient" to use someone else's ID card. And if you really wanted to be pedantic, your friend - if they have allowed you to use their card - has committed conspiracy to commit fraud by false representation...
Personally, if I were you, I'd pay up the money and brush up on UK law.