My advise is ' don't pay' more importantly don't admit that you where driving the car. If you pay you will never win an appeal as it is them that consider any appeals. This is also an admission of guilt. I assume you are the registered keeper of the vehicle !
They will write to the registered keeper and ask that person to pay the PCN. They can get the registered keeper information from the DVLA as they are members of the BPA) The registered keeper is under no legal obligation to give any details of who was driving the car at the time of the trespass. Only a local authority or police can demand this information.
Write back to them staying that although you are the registered keeper you where not driving the car on that occassion and they should write to the person who was. They will then ask for the drivers details and you reply with ' I am under no obligation to give this information.
They will write to you again making threats of court action but they can only issue a civil action against the person who did the trespass and not the registered keeper.
On the 3rd letter you send to them issue a stern warning. Say that you will charge them £50 for each letter you send to them but the first 3 letters are free but a fourth letter from them will trigger a charge of £50 for each letter and will signal their acceptance of your terms and conditions. State that this is fair under the unfair terms act and that you will Persue them for harassment should more letters arrive from them on this matter.
If you are caught on camera or have admitted the trespass then they have you bang to rights so pay up and get the discount.
I have used this method a few times and its worked every time.
I have sample letters for you which I can send privately.
Don't worry about credit reference agencies getting involved. They can't. Do your research on the Internet to clarify what i have said on here.