From the Ramblers' Association website:
"Reports should be made to the relevant highway authority (county, unitary, metropolitan, London, national park) in England or Wales. Mark your letter for the attention of the 'Public Rights of Way Officer'."
My personal view is that if I was just walking in the countryside, not caching, I would ignore illegal denial and carry on. If I was caching, though, I'd probably leave the cache for the time being, on the offchance that I might be discovered at the cache site and then that would probably be a cache removed for good. Either way, though, I'd report the illegal denial.
Do not go gentle into that good night.
Rage, rage against the dying of the light. (Dylan Thomas)