Would you just pay it then?
I don't think b789 is saying that. In general, the role of this forum is to provide you with advice on your options. Which decision you take is your choice, as it's your money on the line.
Parking companies reject the vast majority of appeals they receive - they don't make any money from cancelling parking charges after all. With this being an IPC member, there's also little chance of the IAS upholding your appeal, so if you are intending to fight this, you will need to be prepared for the possibility of defending a court claim.
If you're fighting this, we could do with seeing photos of the signage at the site. The Google Street View you provided shows no relevant signage, but those images are from 2022, so up-to-date evidence would be required.