In Dartmoor National Park, wild camping is allowed for free in certain designated areas. However, you must follow the Dartmoor Wild Camping Code, which includes guidelines like pitching your tent late and leaving early, staying no more than two nights in one spot, and camping at least 100 meters away from roads and other facilities. You can find more information and a map of the designated wild camping areas on the Dartmoor National Park Authority's wild camping page.
For those who prefer more amenities, there are also a number of paid campsites throughout Dartmoor National Park that offer facilities like toilets, showers, and electrical hookups.