I have been buying a weekly rail ticket for like 15 years. Lost photo card years ago and never got it replaced as never knew you actually needed one. I have shown my weekly ticket to inspectors countless times and never had a problem.
Tonight at London bridge one inspector decided he was gonna be that guy. He told me my ticket was not valid and the photo card gives me discount. I said this is not true at all. It's the same standard price, I said why has no inspector ever told me I need photo card over the last 15 years.
Guy wouldn't budge and now I expect to see a letter come to my door issuing a fine. I honestly don't think I have done anything wrong. My ticket is valid. I haven't tried cheating the system for a cheaper ticket. I know I won't win the appeal.
Really this ought to be in the Disputes & Prosecutions section, but here's my view.
Have you received a Penalty Fare? If so, you might want to consider paying it. I wouldn't normally suggest this - but in this particular case, paying the Penalty Fare, and then promptly rectifying the situation by getting a new photocard, is probably going to be the least stressful resolution, by far.
He told me my ticket was not valid
This is true - you need a photocard with virtually all season tickets. Without the photocard your ticket is not valid, as it is not transferable (i.e. it's only valid for use by you) and otherwise you might easily be able to share it.
the photo card gives me discount.
The photocard doesn't give you a discount - it merely entitles you to hold and use the season ticket.
I said this is not true at all.
The ticket inspector was right about your ticket not being valid without the photocard, but he was wrong about it "giving you the discount".
It's the same standard price
The fact that the price is not changed by the presence, or lack, of the photocard, is neither here nor there.
now I expect to see a letter come to my door issuing a fine.
Train companies are not a Court so they cannot impose any fine. Any Penalty Fare issued is given on-the-spot. If you don't get a Penalty Fare then it's more likely you'll receive a letter where they ask for your version of events, possibly with an offer of out of Court settlement enclosed (perhaps in a subsequent letter).
I honestly don't think I have done anything wrong.
Morally, I don't think you've done anything wrong. But that's, again, neither here nor there. What matters is whether you are in the wrong in the eyes of the law.
I haven't tried cheating the system for a cheaper ticket.
That may well be the case - but it is virtually impossible for the train company to know you are telling the truth. For all they know you could be one of several one/two day a week commuters who shares that season ticket. They have no way of identifying that you're the person the season ticket was issued to without the photocard.
I know I won't win the appeal.
You may or may not win an appeal for a Penalty Fare. However, in many respects this is immaterial - a Penalty Fare is a form of out of Court settlement if you like, and if the train company doesn't like the fact that an appeal has succeeded, they can still take you to Court (whether or not they actually have a case).
Whether or not, strictly speaking, you have done anything wrong (or rather, whether you are liable in any way for any wrongdoiny) is debatable really - whilst it may be the case that any Penalty Fare you have been given is enforceable, it is unheard of for train companies to take people to County Court for unpaid Penalty Fares. Instead, they would generally pursue the matter as a private (criminal) prosecution in the Magistrates' Court.
There are two relevant laws you should be aware of here. The first is Railway Byelaw 18. This purports to make it an offence to board a train without a valid ticket, or to fail to hand over a ticket once onboard or having travelled, where you have had an opportunity to buy a ticket before boarding. I say "purports" because I personally am in no doubt that there are a variety of technical and legal reasons why the Byelaw is itself unlawful. Whether or not you'd want to defend any Byelaw prosecution on that basis is something only you can decide - there are plenty of recent threads in the D&P section which mention this matter. But if you don't want to raise a defence under this element of unlawfulness then I don't see any other defence - a season ticket is unquestionably invalid without the photocard and whether or not you knew that is immaterial. The only saving grace would be if you couldn't have bought a ticket at your boarding station but this seems very unlikely to me.
The other relevant law is Section 5(3)(a) of the Regulation of Railways Act 1889 (known as RoRA). This makes it an offence to fail to pay the fare due when travelling by the railway, if you have intent to avoid payment of the correct fare. If you were to be prosecuted under RoRA it is a more serious issue, as this carries an unspent criminal conviction for one year, which a Byelaw conviction wouldn't. However it would also be harder to prove - as, it you genuinely didn't know you needed a photocard (this is something which others may not believe you on though), and if there was no pecuniary advantage to what you did, there would be no offence committed.
(Arguably, even if there is an intent to avoid payment of the fare, that would not constitute an offence here since the correct fare has undoubtedly been paid - it is just that the entitlement to travel could not be proven at the time of inspection.)
It is not clear what train company is involved, and equally it is not clear whether you have been issued with a Penalty Fare or have instead had your details taken for consideration for prosecution. But one thing is clear - this is not something that will simply go away by itself, and though you may have been innocent and unwitting in what you did, it will cause you a considerable amount of hassle in the near future.