That is a very good question, we have "off road sites" where you can pay and play, but more are for 4x4's (mainly landrovers) most don't allow quads/atvs. I know the owners of a few as I also off road Jeeps, and they say I can use them with a quad.
We also have what are know as Green Lanes. These are old public by-ways, open to vehicles. There are many battles going on to get the "usage" changed so that they cannot by used by motorised vehicles.
I live not too far away from the Peak Park, and they are trying to keep everyone happy, but get loads of grief off walkers about 4x4's and trials bikes (and a few quads). As always its a few thats spoil it for the majority.
I often joke that I was born in the wrong country, I love quads, atv's, Jeeps, 4x4's, and the terrain and countryside to use them on, all the things I guess you guys have as part of everyday life? Where do you take your quad? I was going to look into the different parts of America and see if there's anywhere I could may be have a trip to and go out on quads etc...any ideas?