not sure it is! Was having a great time a few years back in one, with some mates, got dobbed in and lectured at great length by the rozzers, and when I questioned their right to interfere they quoted all manner of legislation that would employ if I continued in the course of action.. Didnt risk it at the time!
I used to have one of those animal double cab pickups, and living in the new forest, had masses of fun in that too, winter, spring, snow, floods, you name it!