It is the natural habitat of the Challenger 2 Tank. The British Army constant attempts to capture challenger 2s to go fight in it's wars. It commonly participates in "NATO Excercises", which are more like Hunting expeditions by the British in their attempt to capture more Challenger 2s.
Stonehenge, is part of Salisbury Plain and is a place of worship for the local Challenger 2s, contrary to popular belief that it was built by Aliens. It has been classified as a World Heritage Site by the United Nations, but nobody gives a damn.
It is the only part of Wiltshire to exist. If it doesn't, the British Army wouldn't have Challenger 2s. Since the British army have Challenger 2s, Salisbury Plain exsists. QED