Incidentally, CPS "guidance" to fare evasion, which, as good practice, TOCs and other companies should aim to meet, (although not compulsory to do so), states that in cases like this:
Crown Prosecution Service said:
Where tickets have been altered or defaced consider a charge under the Forgery and Counterfeiting Act 1981.
Link
From the details on this forum, if I was the prosecuting authority, I would bring a charge under the FCA 1981.
Section (1) The offence of forgery.
A person is guilty of forgery if he makes a false instrument, with the intention that he or another shall use it to induce somebody to accept it as genuine, and by reason of so accepting it to do or not to do some act to his own or any other person’s prejudice.
Whilst at least three offences under Railway Byelaws, i.e. 18(1), 20(1) & 20(2) have been breached, the perceived severity of altering a ticket, (obtaining services dishonestly), usually causes a prosecutor to consider more serious criminal (and civil) legislation.
If a TOC is going to prosecute, I would say Section 5(3a) Regulation of Railways Act 1889.
If CPS (BTP) is passed the case, I would say Section 1 FCA 1981.
I think regardless of what charges are brought, a TOC is likely to seek redress in the amount of the (altered) season tickets perceived value.
Could you tell me:
1) Who stopped & cautioned you? (Railway staff, police etc)
2) What is the value of the season ticket you altered?
3) Have you been previously came to the attention of authorities for any similar offences?
4) Did you fully co-operate with the person who cautioned you, and admit liability immediately?