It is not a "legal letter'. It is a debt recovery letter which you can safely ignore. Debt collectors are powerless to do anything except to try and persuade the low-hanging fruit on the gullible tree to pay up out of ignorance and fear. Never, ever, enter into any communication with a powerless debt collector. Ignore.
You have been advised that the Penalty Notice is fake. It is a criminal attempt to defraud you.
If it were a real Penalty Notice issued under Railway Bylaws, the ONLY way they can enforce it is through the Magistrates court by laying an information. They can't, because they are not the authority that can prosecute this. Even if they were, they would not receive a penny of any fine that would be imposed by the court, assuming they were able to prove beyond a reasonable doubt that you were the "owner" of the vehicle. How do you suppose they can prove that? There is no official register of vehicle "owners" and they cannot assume that you are the owner because you are the registered keeper because if you care to inspect the front of your V5C, it says in bold white letters that the V5C is not proof of ownership.
Now they are threatening to try and recover the "debt" through the civil procedure as a breach of contract. They can't have it both ways.
Personally, if I were to ever receive one of those Penalty Notices, I would report them to the police under the Fraud Act 2006, section 2.
You are not merely reporting a civil dispute about a parking ticket—you are alleging that:
• A known private company (e.g. APCOA or SABA)
• Has issued what purport to be Penalty Notices
• Using the language, structure and implied authority of statutory enforcement
• But in fact, these are not statutory fines and no legal power exists to demand payment in this way
• There is an intention to deceive recipients into paying under the false belief that this is a criminal or statutory penalty
• This aligns with the criminal offence of fraud by false representation (Fraud Act 2006, s.2) and possibly even blackmail (Theft Act 1968, s.21), particularly if there is an implicit or explicit threat of legal or financial consequences unless payment is made.