I'm sure the could - and to a large extent that is what they have done.
They seem less keen on reaching into their pockets and finding £1.7B to compensate the airlines however.
The ICAO arrangements state 'no ash'. Which is obviously a pretty silly requirement given the number of eruptions taking place every day.
Possibly NATS risk analysis expertise could have identified the risks of closing huge areas of airspace with little detailed knowledge of ash levels and virtually no technical justification, simply risk aversion?