My general advice about the legality and rules for sleeping wild are that if you are polite and friendly, spend only one night in a place, camp discreetly, and leave absolutely no trace when you depart, then you’ll be fine.
The next step of responsible wild camping would include the advice always offered in books and articles, yet in reality utterly impractical: ask permission from the landowner. Trespassing is naughty, of course. (Although, there was once a time, long ago, when rivers and woods and hills were not ‘owned’.) But annoying landowners really is a terrible thing to do if you care about wild camping being allowed / tolerated. So always bear that in mind.
Far more useful, in my opinion, is to take responsibility for your own actions. Pay attention to Natural England, National Parks, AONB’s, SSSI’ms, Open Access and CROW land. This map is helpful for delving deeper into land use. Also bird nesting season and salmon spawning times will impact where and when you can bivvy.
Bear in mind that as we destroy nature, we destroy ourselves. Or, more simply, if you go sleep in the wild because you love it, but then leave rubbish and fire circles and wreck the place then I may be forced to momentarily shrug off my mild-mannered mask and punch you in the face.
As always I would welcome comments, below, from those of you who know more than me about matters like this!
Una validiores sumus.