Haven't there been a number of people on here previously saying that they don't work legally, and you can safely ignore the requests for payment?
(Whether that's correct or not I have no idea.)
Sort of, but it's more nuanced than that.
In England & Wales, the Protection of Freedoms Act allows transferring the liability of a driver's breach of contract to the registered keeper of the vehicle involved in the breach. So if you don't pay for your car parking in a private car park in England or in Wales, the car park operator can get the registered keeper's details and take them all the way to county claims court, get a CCJ against the registered keeper, and enforce it with court bailiffs if needed.
In Scotland and in Northern Ireland, this legislation doesn't apply, which is why you'll see people often saying you can just bin car park charge letters in Scotland for example. The car park operator can get the registered keeper's details from DVLA like in England and Wales, but the only person liable for breaching the contract is the driver. If the registered keeper doesn't identify the driver, it becomes infeasible to track down who was driving and pursue the charge. If you do it often enough though then the balance does tip back towards the operator finding it worthwhile tracking down the driver as there's more money at stake.
The Protection of Freedoms Act is quite specific though in saying that it only applies to 'relevant land', which is land where parking isn't covered by other statutes. This means it doesn't apply to say council car parks (though the law for council car parks is stricter and can be enforced against the registered keeper even in Scotland). It also means it doesn't apply to most airport car parks because these tend to be governed by bylaws. So if you very sparingly breach major airports' car parking terms, don't reply to their letters to identify the driver, and don't pay up, there's a fair chance you'll get away with it.
The caution here is that you're taking the risk that the operator won't invest the time to identify the driver through some other means. The charge is still valid as the driver has breached the contract, and the debt therefore still exists. If the operator manages to identify the driver somehow, the driver can expect the usual late payment and legal fees etc to be added on if it gets taken further.