That isn't the case.
There are lots of web-sites and forums where all sorts of claims are made about defences for failing to pay for parking/overstaying - some of them have been repeated on here - but the law is quite clear. You can be charged for parking on private land as long as the terms of the contract are made clear - usually by the display of a notice or notices. You can be asked to pay damages should you overstay if those were allowed for in the contract . The damages must be proportionate. The courts have upheld claims for damages from private parking providers. IIRC, one judge effectively said it should be calculated on several multiples of the daily parking rate - I'll see if I can find the reference.
Railway car parks are slightly different. Although these are mainly operated by private parking firms, they are normally operated on behalf of the relevant rail operator and are subject to specific bye-laws.
You cannot enter into a contract simply by driving past a sign - whatever the sign says.
A contract with a private person has to go through stages, offer, consideration, acceptance, and often a cooling off period.
The signs are, at best, no more than an offer.
The alternative is to treat the parking as a trespass or tort, to quote: "a wrongful act or an infringement of a right (other than under contract) leading to civil legal liability".
It matters not, given there is no penalty at court for either, simply damages.
If parking is free, as in my case under discussion, it's difficult to see what damages can be reasonably awarded.
As regards courts upholding claims, they may have done but one might have thought some cases would be immediately apparent online.
The parking companies like people to believe this as part of their intimidation tactics, so the parking companies would trumpet the cases, even if no one else did.
I did find a county court case from a few years ago which went against the motorist, but he was parking regularly all day, partly blocking the entrance to a church.
What I have never found is a court case involving a private ticket for an over stay of a few minutes.
