If I remember rightly if it is actual National Park, as opposed to National Trust access land, you can camp anywhere.

I'm afraid this advice is almost on a par with "In England, it is customary to greet a lady by grasping her breasts with both hands and saying 'cor, what a bunch'"

Most of the national parks are farmed land or built up. In the areas that are moorland, wild camping is sometimes tolerated, but as likely as not you will get an irate landowner or gamekeeper waking you at 5am to complain. Some moorland areas are owned by the National Trust, and there's usually no one around to enforce their ownership rights.

The best advice is to get a list of campsites from the tourist board, and if you want to use b&b or camping barns, book ahead as they can be busy during the summer.