You can legally wild camp almost anywhere in Scotland, meaning that the Highlands are a veritable playground for campers. Much of Dartmoor, in South West England, also allows wild camping. (This map shows where.) However, there are three firing ranges to avoid, and you’re not allowed to pitch your tent right by the road … but you surely didn’t come to the moors for a view of tarmac, anyway.
Outside of these areas, wild camping is only permitted with the consent of the landowner; however, that doesn’t mean that wild camping is out of the question. There are actually many socially-accepted wild camping spots across the UK, but stays are generally limited to 1-2 nights, so don’t take up permanent residence. Beyond that—and provided that you’re a responsible camper and leave no trace—there are plenty of remote spots where you're unlikely to have problems if you pitch your tent late and leave early.
Insider tip: Apps such as VanSecrets give great recommendations of wild camping spots.