This was shared on another thread - I hope they don't mind me sharing but I found it a really informative read and seems like where government are getting guidance from regarding the 5 point threshold. It explains how school opening fits in and case numbers required to move to next phase etc. It's helped me get clarity of the plan even if government aren't sharing!
institute.global/sites/default/files/inline-files/A%20Sustainable%20Exit%20Strategy%2C%20Tony%20Blair%20Institute%20for%20Global%20Change.pdf