The whole basis for wheel clampers charging a release fee is in contract law.
The idea is that the land owner (or their agents) offer a contract - by way of a big sign displayed.
The contracts offers to let you do something that would otherwise be unlawful, such as drive on to their land and leave your car for a period of time. Without the contract it would be a civil offence of trespass.
In return you agree to certain terms, usually the payment of a fee and a requirement to display a ticket or some such, but also, usually, to consenting to your car being clamped and payment of a release fee.
The contract is important as it (a) lets you do something you could not lawfully do (without it you are trespassing on the land), and (b) lets them do something they could not lawfully do (attaching a wheel clamp without your consent is trespass on your car).
The issue, usually, is that the penalty is disproportionate, in some cases you may not actually have seen the sign and agreed the terms, and that by clamping your car they put you under duress to force your to pay the charges. It is just like extortion.
The idea is that if you park then you have agreed the terms. Your action makes the contract.
So, here is an idea :-)
Make them a contract offer too! In this case a very clear sticker in the car.
Now, the logic is simple. They have no right to stick things to your car. So you offer them a contract. You let them do that in return for money.
The arguments for this are identical to the arguments for the clamping. If they put a sticker on the car (one that says do not attempt to move the car for example) then they must have agreed the terms. The fact that they have to put one on the car for safety reasons is not really the issue. They don't have to clamp the car just as you do not have to park there. They choose to clamp the car and attach a sticker knowing the contractual consequences.
Then, when you pay the release fee you take court action for return of the release fee and the money for sticker removal at the same time.
In theory, any argument they have about a contract formed by actions and a clear sign and so on you can use the exact same argument as to why they owe for the sticker removal fee. Any argument they have against the sticker removal fee you can use for defeating their claims for the clamp release fee. The idea is to make the legal arguments symmetric.
That being the case you either lose the clamping fee but win the sticker release fee, or you win the clamping fee back!