Terrain for Offroad Driving in SE England

Mercedes invited us to drive the ML350/270/500 vehicles in "true"
offroad conditions over the last weekend and everyone including me who
are using their cars in the Tesco run and the School run were quite
impressed with what the vehicle is capable of doing. Unfortunately
this was done on a private property and one would probably have to pay
a lot of money to use it.

Somebody mentioned that there are maps available that show unpaved
roads in the country where one could drive the 4X4's without breaking
some sort of law.

Anyone knows anything about this?

Thanks

Mehmet Artun