NATS are an ANSP (the only privately owned ANSP in Europe) and operate on a licence issued by HMG (well CAA). The government are signed up to the ICAO guidance previously refered to in this thread which states ANY ash, no fly. NATS complied with the conditions of the licence and closed the airspace it operates (on behalf of HMG), as did most of the North European ANSPs. The ball was then in governments courts to change the guidance in consultation with various other bodies. This it eventually did once they'd gathered more facts/data and it was then up to HMG to announce the changes.
Cynics amongst us may say that HMG hid behind NATS skirts so as to make it appear that NATS were the baddies of the piece and HMG rode the white charger to 'save' the airline industry. In reality everyone did exactly what they were supposed to do.
The airlines were the only ones who could provide the data to set the limits on ash levels. Until a week ago they were unwilling to do so and unwilling to set a 'safe' limit, probably becuase 'safe' is a relative thing.
When its comes to things like this its the CEO's responsibility to ensure that a business keeps function and pro-actively prevent a business disaster happening. It seems they collectively decided to avoid the issue.