To camp anywhere requires the permission of the owner or tenant of the land and some payment may be necessary. Common and unfenced moorland is mostly privately owned and subject to the rights of the commoners. Those who drive or park on such land may be liable to prosecution. A farmer or forester is sometimes willing to permit camping on his land but any such 'casual' site should not be used for more than a limited number of days per year. All sites should be left clean and tidy. It is important that rubbish should be left in the places provided or better still, taken home.
You can find a very comprehensive guide to Camping over at Wonderful Wellies.