I have been a few places with some riders and we have been quite far and have only come across one which is Thriftwood in Brentwood, Essex. You do have to pay £2.40 to ride but it is really good. It has got logs, Beams, Pallets,Cable drums, A frames, you name it. And if you are lucky you can sneak in and not pay and they don't even notice. Hope that helps.