If I were ever to be clamped at a service station I would kick up such a stink with the manager of the service station it WOULD be released for no charge. Either that or the stihl saw/boltcutters would come into play. I would not under any circumstances (as long as I had been a paying customer) pay any de clamping fee or any so called "Parking Charge Notice" UNLESS it was from the Police. NEVER!!
To be honest, I have only rarely seen cars having been clamped at the services. It is usually a charge notice put on the windscreen. I have even seen an "old banger" still there on a revisit some days later, with no clamp.
I am sure that in the past it has been mostly bluff, but with them having "sold the rights" for enforcement to the type of contractors that DO work on other private car parks, and DO clamp freely, I suspect that it is no longer a bluff, and that any charge will be followed up. The fees also exceed guidelines laid down by the voluntary code, which is a suggestion that the companies involved are free to do what they like, and have been doing so in earnest recently before they are finally put out of business in England as well as Scotland.
It is this persistent behaviour that has made the government outlaw clamping despite objections from landowners that they will be left with no way to protect their carparks from a "free for all". Had these companies behaved fairly, they would NOT have faced this change in the law, and may even have escaped it in Scotland.
At least without clamping and towing, the motorist cannot be held to ransom to pay on the spot, but can go away and mount a challenge to the charge if it is enforced through the courts (which is probably unlikely).
I suspect services will go back to managing their own carparks after clamping is banned, as the companies will no longer pay service station operators for the rights to operate parking control. It will be the other way around, with them asking to be paid to patrol the carparks. Only in peak times such as August bank holiday and the school summer holiday do they encounter problems with fitting new arrivals into the car parks, which is the only situation in which they can LOSE overall revenue from new visitors going elsewhere as car park "blocking" visitors do not spend enough per hour to make it up.
I also recall that in the old days most of the parking charge was given back as a restaurant voucher so that if you had stopped to rest and eat THEIR food, you paid very little for the parking, and the charge was to discourage non-paying customers from leaving their car there and spending money elsewhere, such as by bringing a picnic, or cooking in their caravan.
The original purpose of the charge was to ensure that spaces were always available to motorists wanting to pull in, and so only needed to be enforced for a fraction of the time.
There is NO danger of the car park being overfilled at 3am, but the new companies are out ticketing at this time, even in a SNOWSTORM!! This is not just greed, but a SAFETY issue, as it is forcing motorists to drive on when it is unsafe to do so due to the weather or tiredness, with those putting safety first being charged £10 or more for NOT leaving within the 2 hours regardless of conditions or tiredness. The highways agency just does not see this, and despite constantly running motorway campaigns aimed at getting drivers NOT to forge ahead if tired, or conditions are unsafe, they allow the operators of service stations to use a punitive charge to pressure motorists to move on after 2 hours, even if there is plenty of space available for them to stay, and new arrivals to park up.
This system also means that other laybys are stuffed with lorries, leaving nowhere for others to park up short term if needed. This is because the haulage industry is on it's knees already over high diesel prices and the inability to pass these on to customers because of competition from abroad using cheap diesel. They also save money by taking short cuts through small towns and villages, rather than taking the wide roads and dual carriageways that may get them there faster, but cost more in fuel. One notorious shortcut is used one way just to avoid the toll on the Severn bridge, as it is actually cheaper to fight through a few small villages to get into Wales from further up the M5/M6 than to go over the bridge. It's free to leave Wales, so it's not a problem the other way (if anything, this is the wrong way around

).