Man of Honour
Clamping on private land by parking companies has been illegal for a few years now in England, unless under the authority of someone like the DVLA/Council/Police.However, I would have thought that by choosing to park on "Private" land and refusing to agree to the T&C's, you may not have agreed to the contract but would instead be committing a "Trespass to Land" br parking there without the landowners permission.
What the consequences of this would be, I do not know. But I think you could still be clamped and have life made difficult for you.