NATS has been the 'face' to the media of the UK's decision to close its airspace in the face of this unprecedented event. I would sugest that this has been been politically expedient for the higher echelons of govenment and, perhaps, the airlines (initially) to have NATS as the 'fall' guy as and when blame begins to be bandied around at a later date as will inevitable happen.
I am of the opinion that there has been massive over-kill in this matter and that tactical discretion to fly or not should have been made available to the operators.
It was always held that air traffic control provides a 'service'. In my experience the ATC system has become more 'executive' than 'advisory' over the past 25 years but that is a topic for another debate. However, the service that NATS and the Met office should have provided was constantly updated information on the location and concentrations of the ash.
Of course, NATS states that it has reacted to ICAO procedures and this whole affair is reminiscent of the 'liquids' ban. It happened to the UK and the rest of the world has been forced to comply irrespective of the threat (or complete lack of it).
I would suggest that there should be a complete re-think/re-write of this ICAO procedure as and when the crisis is over. The economic consequences of this blanket, unthinking policy has been dire. Safety, of course, is paramount but I am positive that many air services could have been operated since Thursday with no risk to crew or passengers whatsoever.