From the Daily Mail: New ash cloud could delay re-opening of London airports. We have this gem: "Critics said the agency used a scientific model based on 'probability' rather than fact to forecast the spread of the ash cloud." See the Telegraph as well.
What else are they supposed to do? The agency here -- the Met Office, which is the national weather service of the UK -- doesn't know what the ash cloud is going to do. If they waited to see what the cloud does, the planes would already be in the air. It would be too late.