In reply to Gokul:
Don't know why you got downvoted so much, you weren't suggesting people give away their "secret"spots, just suitable places to camp. More than most consider when coming here.
You won't have any trouble in Glencoe, just pick somewhere suitable, don't block gates, or park over anyone's livestock. And being from the US you'll know about leaving things tidy, ie, bury your feces and carry out your toilet paper.
Camping is permissible in Scotland, people are free to enjoy the landscape given they adhere to a common sense code respecting the outdoors.
Down south not so much. You'll want to stay in campsites. But the one in Bosherston in Pembroke is great as suggested, and in Cornwall there is a farmer in Zennor who usually opens a field up for a fiver/night, same down the road in Morvah. Hope that helps and you have a good trip.