DA50driver....
What is the Met offices evidence that something bad will happen
It's pretty simple. They have none. The Met Office are asked to forecast the movement and intensity of the the ash in accordance with the definitions in the ICAO EUR/NAT Volcanic Ash Contingency Plan (ICAO EUR Doc 19 and NAT Doc 06 Part II, December 2010 edition) They do this using one of the most advanced dispersion models in the world. It is then up to the CAA, manufacturers and operators to decide what's safe or not. This idea that the Met Office are sitting staring at their computers deciding what will damage aircraft engines and then closing airspace is ridiculous. That is WAAAAAAAY outside their remit.
As an aside. I'd imagine if the airlines coughed up a few quid then the dispersion model could be better, observations of ash could be better, and disruption could be more justified.