Just a thought on the issuance of the ticket. Assuming that the OP could pinpoint pretty much the exact time, date, payment method and place of purchase, down to the booth/machine etc etc... is it possible for the railway to find a record of that ticket sale and provide a duplicate upon request?
You'll need to bear with me here, because I understand the issue in this case - travelling without a valid ticket. I also understand that purchasing a ticket doesn't mean that it was for the intended person. I'm just testing the plausibility that the OP could link the purchased season ticket to their ownership. Given that their evidence bundle will rely on year's of weekly purchases, a pattern can be formed, and that they had indeed purchased a ticket.
I'm not suggesting that the OP takes this as advice on the route to go, I'm simply curious since this whole situation apparently hangs on the OP being unable to produce the ticket. A pattern of similar purchasing behavior, and acting in good faith on the instructions of a railway official, could be enough to tip the balance.
Obviously at this stage, the OP should get a solicitor, and have them argue it out in the courtroom with the facts, and the documentation that they have, rather than representing themselves - that is a daunting thing to do, and generally doesn't end well.