Could it be that most schools reject "John Major's GM dreams" for the simple reason that opting out really means opting in? By opting out of the local authority's already limited control, schools are opting in to centralised control. By not opting out, they are choosing the arguably more balanced control of both the Government and the local education authority, rather than opting in to whatever any government of the day might decide to impose.
If this is so, the majority of our schools are to be commended for their wise wariness of what they might be opting in to by opting out.
8 Medina Court