I don't think they have any legal weight at least insofar as they are not defined as regulatory signs. However, as I'm sure you are aware, they are referenced within the Rules of the Road as follows:
Variable Message Signs (VMS)
These signs provide information in an electronic format and are designed to inform you of a range of issues relating to roads, roadworks and road safety. The content of the sign will change, depending on the situation. You should pay particular attention to these signs and messages.