Sounds to me like they have clamped the car to ensure you can't remove it before it's uplifted, so they can then charge you for the operation, storage, retrieval, etc. without you having a chance to collect it. Why the council would order them to immobilise a car they want shot of beats me, although they do have the authority to do so despite private clamping having been outlawed. They can also do it on private land under certain circumstances. Probably best to contact the police to determine if it is lawful in this case.
Is the car obviously unroadworthy? If so, they may have clamped it to prevent you driving it.
If the land it's on is yours you could presumably deny them access, although you'd have to sleep there, as said. Can you block the entrance in such a way that they would have to cause damage to enter?
You could remove the unclamped wheels, even the hubs as well, if you like, as they'll probably be expecting to roll it onto a truck. They could just pick it up with a Hi-Ab, of course, and no doubt that would attract an additional charge that you would, naturally, be liable for.
I would perhaps photograph the car and remind them that you will be holding them responsible for any damage should they touch it.
Police is probably the first port of call, though.