Can we put the 'over reaction', 'Backtracking', 'I can't see any dust' posts to bed please?
ATCOs, and ANSPs are not allowed to give IFR clearances into areas where Volcanic Ash activity is forecast. End of.
They come about this information from the met office and vulcanologists. It is updated every 6 hours and everyone is briefed - any less time is unreasonable to expect decent data, or for airlines and passengers to be given notice that their flight will be OK.
There may well be some scope for flights to fly VFR below then climb IFR when clear of the forecast areas... However that implies a willingness by the airlines to fly under any area affected with Ash Cloud (remember gravity anyone) and also the willingness of airlines to burn lots of extra fuel in doing so.
Just why would airlines, who are struggling already, want to operate a flight originating from under a risk area using methods that are going to decimate most if not all profit on marginal profit routes?
These airlines are the same ones that took such measures as reducing in-flight magazine sizes etc in a bid to reduce weight and therefore fuel burn.
As someone said much earlier, the usually vocal CEOs of certain LoCo airlines are not uttering any complaints about what is happening... that is telling in itself.
This action may well be overkill - but unless you can say for certain, would you take the risk if you did not have to?