If you look at the number of cases per 100,000 the only place outside the SE (and not on the list of places where all schools will remain closed) with more than 800 cases per 100,000 are Brampton & Irthing an Penrith Outer (Both Cumbria), Brierfield & Reedley (Burnley), Yapton & Climping (Arun) and Denmead & Southwick (Portsmouth).
I would be nervous in any of these areas sending my son back to school.
I personally think that if you make this a firm cut off point, it would at least give some certainity and sense of consistency.
There are not many other places at the upper end of the 400 - 800 category (and those that are are in Cumbria, around Hartlepool) at the moment.
If T4 is to work with schools open then it shouldn't be a problem and we won't need to put other places into T4+ and shut the schools, so this cut off point is very reasonable... However if T4 with schools open doesn't work then those numbers are going to increase and we shouldn't be reopening them...