And why can`t the template letter which notifies the parking company that I as the owner does not want to nominate the driver work in this instance?
Ownership is irrelevant here, what matters is who was driving, and who the Registered Keeper of the vehicle is.
Schedule 4 of the Protection of Freedoms Act (PoFA) provides a mechanism for parking companies to hold the registered keeper of a vehicle liable for unpaid parking charges, in situations where they do not know who was driving. It requires them to abide by certain requirements to do so - their notices must be sent within certain timeframes, and contain certain wording, as set out in the act.
They often get this wrong, in which case, the sort of templates you are referring to can be used to point out that the keeper cannot be held liable. In this case however, CPM seem to have complied with the requirements, which means that if they do not know who was driving, they can seek to hold you liable as the registered keeper.
Should I draft something up regarding the shoddy signage as mentioned by @DWMB2 and if yes, is there any template for that?
If it was me and I was planning to fight the charge, I would be appealing - I take the view that if you have a reason you believe the money is not owed, it is sensible to point this out. They won't accept the appeal (no money to be made from doing that), but it sets out your position, and shows you intend to fight the matter. There isn't a template, as the appeal is based on the circumstances of this specific case. You can keep it simple, and just point out that the size, content, and positioning of the signage is insufficient to bring to the attention of the driver the alleged terms and conditions on offer.