Hi, what shall I say exactly? I don't know what an SAR is?

Hi, I have received a letter from TFL for failing to produce a valid ticket, pass or photocard for my journey the London Underground. have asked for my side of the story and have been advised that legal proceedings may be taken against me. What happened is that I was on the tube then an inspector came on and I gave him the Oyster card that I had used for the journey. He scanned it and asked to see the railcard attached so I gave him my student railcard. He then told me that there is a greater discount on the card and asked if I have another railcard. I said that I don't. The inspector revealed that it was a privilege discount which is 75% and that I should have a privilege card to be entitled to it. he then cautioned me and started questioning me and I explained that I bought the blue Oyster card and then asked a staff member to attach my 16-25 student railcard to it. The inspector withdrew the oyster but returned my student railcard. I was told that even though it may have been a staff mistake where the wrong discount was attached, TFL can still take action against me for using a discount that I wasn't eligible for and benefitting from it. Please advise me on how to respond to their letter. Should I argue that it is not my fault that the staff made a mistake and I am not responsible to correct it. It was the staff's responsibility to check the right discount was given and there was no reason for me to assume they could not properly perform their job. Also, I could have only found out that the wrong discount was given after using the oyster and then calculating the fare I was charged, so travelling without a valid photocard was unavoidable. I didn't know that privilege discount existed, so even if I noticed that I had a greater discount, I would have still not known what is was for me to go and report it. I would appreciate any help on how to respond to this to get the best outcome possible.