1 - the barrier is across a road at the boundary.
2 - these particular roads are permissive, they are on MOD land but these particular routes and certain footpaths and bridleways elsewhere on Salisbury Plain  have permissive use by the public subject to restrictions by the MOD which are usually but not always posted on the MOD website @ https://www.gov.uk/government/publications/salisbury-plain-training-area-spta-newsletter .