If you lose at POPLA, they'll engage debt collectors. These debt collectors are powerless and will simply send scary-sounding letters. They will not be turning up at your house and you can ignore their letters.
Eventually, the debt collectors will get bored and pass the matter back to the parking company. The parking company may then choose to take court action. If they do, you'll receive a Letter of Claim from their solicitors. If that happens, come back here for advice.
In the meantime, keep an eye on your emails for POPLA's decision. Once you've submitted your comments you should get an email advising you of the estimated wait time. It's usually at least 6 weeks.