I was told once that if there is no speed limit displayed the cameras are off, not sure how true that is.
When last over and using the M25 I was told if the variable speeds are not set (e.g off) then so are the cameras. I went through a craptonne of them at more than 70 and had no flashes.
Whether the same applies to the M6 I have no idea but the theory at least for the M25 is sound.