I am not a lawyer, but I have quite a lot of experience of negotiating contracts and working with lawyers and I have to say I am very skeptical as to how enforceable some of these fines are.
Since these are civil disputes, the parking company has to show that you have agreed to these terms and conditions and an implied contract needs to exist.
If you have no idea about such a penal charge, and it is reasonable to assume that the lay person would have no idea, then no implied contract can exist. Suppose I decide to charge the Amazon delivery guy £100 for walking on my drive - do you think I could get that to stick? No chance.
I would tell them you didn't agree to their terms, and that you won't be paying it. I would NOT ignore it.
If they threaten to take you to court, I'd also not ignore that, I would tell them you will defend any court action. And if they do take you to court (very unlikely imho), I would go and tell it straight to the magistrates who I think would take your side.