That appeal is totally wrong. You identified yourself as the driver. It is not a Penalty Charge Notice. It is a Parking Charge Notice. Big difference.
You have not been accused of committing any offence. This is a civil law matter. You have been issued with a speculative invoice for an alleged breach of contract by an unregulated private parking company. You have not contravened anything.
However, by identifying yourself as the driver, you have thrown away the best chance you had of getting this cancelled. What we need to see is the signage at the location. We need to see the prominence and location of any signs and the actual wording on them. If this occurred after dark, take some photos of the general area in similar lighting conditions, without flash, to show if they are even noticeable or evident. The more illegible, the better for your case.
As this is an IPC member, appealing to the kangaroo court that is the IPA is not worth while. Have you tried Plan A? Do you know who the landowner is? Is it the location that you either dropped off or picked up your passenger? If so, contact them and ask them to have this cancelled. You may want to remind them that it is possible that the location will be blacklisted by taxis and delivery people if they are likely to be subjected to speculative invoices from ex-clampers for doing their jobs at this location.
For now, besides the above Plan A, you will have to weather the storm of useless debt collector letters and wait and see if/when they issue a Letter of Claim/Actual claim. If they do, then come back and show us the LoC/claim and a suitable defence will be suggested.