It will come as no surprise – car parks, Red Lion and all facilities will be closed and over the Solstice. For anyone wanting a solitary walk, the 49 bus from Swindon and Devizes is still running a 2 hourly service (face masks and hand sanitizer not supplied).
heritageaction.wordpress.com/2020/05/18/avebury-closed-for-summer-solstice/
Also applies to Stonehenge.