UKPC has uploaded their evidence and documents. It looks like they do have the permission to issue PCNs on where the motorcycle was parked, though the boundary lines in this image are thick and hard to tell if the vehicle was inside, outside or on the blue boundary line:


They also pointed out that my photos are old and from Google Maps:
"The images the appellant has provided of the signage are taken from google in April 2018 which does not show a true reflection of the signage on site when the parking charge was issued. Please see attached a signage plan showing the signage locations, the entrance signage proof and onsite signage proof. We feel it reasonable to suggest that the driver was advised sufficiently of the terms & conditions of parking on site."
Although the photos they provided are also from ~2 months prior (7th March vs 29 April, when PCN was issued) and some older than that.
Even in their photos, my point around the sign being on the right side of the entrance, and it being practically impossible to read after a right turn to the car park, is evident:

They then add:
"We would also contend that when a motorist enters a car park they should be at the same level of observance as when driving on a public road; When driving on the road, motorists are expected to be aware of signs when travelling at up to 70mph. In a car park, the typical driving speed is on average, much lower. We therefore contend that it is not unreasonable to expect a motorist to note the signs and to subsequently familiarise themselves with these."
Once again, referring to the photo above, as soon as the motorcycle/car completes a right turn, the sign is at a 90 degrees (at best) to their right. Even at a complete stop this is pretty hard to read!
Another point they raise is around the paved area not being designated for "bikes/ bicycles":
"The area the appellant parked in was not an authorised parking bay as there are no clear line markings to outline a bay. This is a paved pedestrian area, therefore they were causing an obstruction to passing pedestrians. This is not a paved area designed for bikes/ bicycles as the the appellant suggest."
The area is absolutely designed for bikes/bicycles; it has ground anchors (just to the left of where the bike is parked). There is no marking to show where the bikes/ bicycles parking area begin and end, and I think it's reasonable to assume that small paved area is for bikes:

Appreciate the photo above is from Google Maps, and perhaps some time ago, but that's exactly how the area still looks like (I was there less than a month ago). They haven't provided any recent photo of this area.
They have provided their contract and some other documents in PDF. Could attach those too, if you guys are interested. Popla email says I have 7 days to add comment to their evidence. Would you say above comments are good to add?