The CAA/NATS would I'm sure have preferred to say something along the lines of "there's ash out there guys'n'gals, check the VAAC as to where it is forecast to be, launch at your discretion", however I'll bet the operators took the view "we're only going if you say its safe to go and if it turns out it wasn't safe then we're coming after you"; the blood-sucking lawyers will have had a field day sorting out where the responsibility rests in the event of some poor sod turning their 747 into a lawn dart.
The CAA have basically said that; the risk is entirely with the operators.
If you read the Guardian article, its explains that the airlines have refused to set a safe operating level precisely becuase that exposes them to liability if things go wrong.
Why airlines resisted setting safe dust level for flights ? until now | World news | The Guardian