As far as I know it’s not illegal unless there are local bylaws which explicitly prohibit it…. problem is in most cases they do and most public car parks have signs up saying that overnight sleeping is prohibited.
There are options to get away with it in more remote areas (here on Dartmoor for example), but it sounds like the size of your vehicles may make those spots inaccessible!
The other thing to bear in mind is that if you’re parked up on a public road, you’re still technically in charge of the vehicle so drink driving rules apply!
For the sake of paying for three nights in a campsite, I’d be inclined not to bother trying to wild camp. Plenty of nice campsites down this way which are right on the beach…