You still have never received ed a "fine". I will give you £100 for every occurrence of the word "fine" you can show us on the paperwork. I'll also throw in £50 for every occurrence of the word "penalty" or "offence" you can show us.
What you received (well, should have received) from Britannia is nothing more than a speculative invoice for an alleged breach of contract by the driver. Britannia are not any form of authority that can issue "fines". They are an unregulated private parking company.
You first have to ask why didn't your receive anything before this useless debt recovery letter which you can safely ignore. In most instances, it is because when you last moved, you may have updated your drivers licence details with the DVLA but you didn't update the V5C registration document. The two are not connected.
Britannia can only access your DVLA data once. If it was not up to date at the time, then all correspondence went to the old address. DCBL will have presumed a credit reference check and found your current address and sent you their scary letter.
Debt collectors are powerless to actually do anything except to try and make the low-hanging fruit on the gullible tree pay up out of ignorance and fear. Never, ever, ever enter into communication with a useless debt collector. Ignore them.
So, is your V5C address up to date? If not, then you need to urgently send a data rectification notice to the DPO at Britannia, instructing them to update their records with your current address for service and to erase the old address. The highlighted words are there for a reason, so use them.
Please let us know the status of your V5C and then we can advise on how to move forward with this. Suffice it to say that if you follow the advice, you won't be paying a penny to Britannia.