The nub of it is that private parking “fines” are extortion, nothing more, nothing less.
In parking on private land you enter a contract with the owner and if you break that contract they have the right in law to try to seek redress from you. It is proper that this right exists because private land is often abused by overnight stayers, pikeys etc.
If I broke the terms of the civil contract and they said to me “OK that’s a fiver, which is a contribution to our costs in maintaining the car park for your benefit” I would pay to get them off my back and because I knew I was in the wrong in some way. However the moment they write me a threatening letter trying to frighten me into paying them £180, they become opportunistic extortionists and they lose any tiny moral high ground they may have held, which is when I will resist out of principle.