If the appeal is rejected then we move on to POPLA. As the NtK is not PoFA compliant and the drivers identity is unknown, POPLA will most likely be successful.
However, we are dealing with a supposedly independent organisation. However, they are a private company and their funding comes from the operators who have to pay for the assessment. This means that it may depend on which POPLA assessor reviews your appeal. It could be one of the morons they employ or it could even be the tea boy. We don't know.
However, if the POPLA appeal were not successful, the decision is not binding on you. There would be a remote possibility that they would try to litigate over the alleged debt but it would never get as far as a hearing for two reasons. First is that they would have no hope in court which is the only truly independent dispute resolution service and secondly, because of the first reason, they would eventually discontinue once they realise that you are not low-hanging fruit on the gullible tree who I going to pay pay them out of ignorance and fear.
One step at a time. No one who is here receiving advice and following it pays a penny to Horizon.