Depends where you are. If you are in the middle of chuffing nowhere (like we were when we walked the pennine way), then noone is really going to come and see you…
Basically, camp somewhere that isnt on a farmers field, and is out of the way of roads and you should be fine – we did this for about two weeks. Generally noone will care if you are just off a footpath a short way, provided as I say it isn’t in a farmers field or anything.
But people are right when it comes to the legality of it – though its tresspassing which is a civil offense isn’t it? So if you don’t do something really stupid you will be fine.