As they have gone to debt collectors, you could just ignore - come back if you receive a Letter of Claim from their solicitors. It's too late for them to consider an appeal. If you search on here for airport drop off cases, you'll see they have no way to recover the charges from you as the keeper of the vehicle.
As an aside, I note your comment that you were away for 3 months. If you are the keeper of a vehicle, and are going to be away for that long, you need to ensure there is some way for you to have your post checked semi-regularly. If this had been a criminal offence such as speeding, rather than a private parking charge, and you hadn't responded for 3 months, you'd now potentially be facing prosecution.
