I can't answer the primary question, but doubt very much if an illegal parking spot negates the readings / penalty notices. They've been operating for years and if this were a loophole, I'm damned sure it would have been exploited long since. The York Council Lendal Bridge and other parking restrictions fiascos are different as councils have to keep to very closely regulated procedures, announcements / communications, layouts, road markings, signs etc. Any deviation in any respect makes the resulting order invalid and fines illegal. Same applies to speed restrictions vis a vis signage and repeater signs where needed and police enforcement action becomes invalid if there is a cock-up such as missing or too far apart repeaters.
North Yorks has never employed fixed cameras and alway used the high-tech mobile vans so I've passed them literally hundreds of time. I'd take issue with
@FrenchieBoy on whether you can spot them far enough off to slow down if needed. It's just another Transit size vehicle in a laybye a half-mile ahead with a very small black window. Given the prevalence of similar looking vans parked in the speed vans' favourite spots, 'speeders' or even plain nervous types would brake approaching some locations I can think of on a near daily basis.
An early and still favourite location in my locale is the Tadcaster bypass on the east-west A64 dual carriageway. In the early days the van (there was only a single example back then) would park on the grass verges at either end of the bypass where the old and new roads diverge. This is illegal and after a year or two, the local authority must have complained because the vans moved to laybyes of which there are several on this bypass. As much of the road is in a deep cutting with long gentle curves, it's ideal for the police as sight-lines are good enough to catch people pretty far out but enough to mask their presence especially for motorists overtaking high sided vans and lorries.
My experience of these vans is that motorists see them when 50 to 100 yards away and almost invariably brake hard (far too late anyway 99% of the time) whether they're breaking the limit or not. Twice now I've been close to near pile-ups, close enough to have been given a severe fright on the aforementioned Tadcaster bypass. This is where a column of faster traffic overtakes a slower one with drivers in the outside lane way too close to each other. The lead driver in the bunch realises the van in the laybye ahead is a speed enforcement vehicle, jams on the anchors, and causes mayhem amongst those following him some weaving into the nearside lane while taking avoiding action. I've been very close to such characters side-swiping me on two occasions over the years. Ironically, the worst outside lane bunching is often caused by the lead vehicle driving at an indicated 69 mph (ie 66 or 67 with most cars' speedometer accuracy) but with more aggressive press-on types behind getting impatient and tailgating. Being well within allowed speeds simply doesn't stop many individuals from braking heavily if taken by surprise. Whether by happenstance or design, I never saw these employed on the nearby A1 north-south trunk road when it was a dual carriageway (or in the remaining dual section between Doncaster and of Ferrybridge). Miles long queues of vehicles close together in both lanes is the norm at busy periods with the outside lot often tailgating each other at 70 or damn close. The mere sight of a speed van would see tens of vehicles rear-end each other and their drivers lose control!
