What "fine"? Why do you keep considering that this is some sort of statutory penalty? Get it in your head that all you received was an invoice from a private company.
Why would you even consider paying an invoice from a private company if you do not agree that is valid? Do not be the low-hanging fruit on the gullible tree that falls for their scam.
They can add as much as they like to the invoice amount. that doesn't mean that you owe it to them. They just hope you are gullible enough to know no better and an easy "mug" for fleecing.
Wait for them to reject the appeal. They are going to reject no matter what you respond with because there is no chance for easy money in it for them by accepting your appeal.
When they reject, they are obliged to provide a POPLA code which you will use to submit a secondary appeal. If they still want to contest your appeal, Britannia must pay a £35 fee to POPLA for the assessment. Even if POPLA don't uphold your appeal, their decision is not binding on you and you do not fold like a pack of cards and pay up.
We will cross that bridge if/when we come to it but no one pays a penny to Britannia if they are here receiving and follow the advice we provide.
