hi,
with it being a pop up i really cant see it being a problem, as the worst they can do is tell you to take it down, if you are going to a campsite then they would charge you for a pitch but im guessing you would be putting it up in woods or on a field etc so as long as you are not trespassing on anyone's property then i think you should be fine, you could always check rspb site see if there's anything on there good luck