I don't know that much about volcanoes, but there is very little information available to Joe Public which indicates that the ash cloud would be a danger. Yes, there's a plume around the Shetlands, but Joe Public living in London who wants to fly from Gatwick to Spain can't see how that affects him.
Presumably some people reading this thread have access to rather more information than is publicly available. If so, could they consider making it available to Joe Public, or do a better job than just saying 'When Eric Moody....'
London is 1,000 miles away from the volcano - so any ash dispersal is likely to be at a relatively steady pace dependent on the wind and presumably moderately predictable based on meteorological forecast, rather than relatively unobservable underground geological events causing the volcano to suddenly increase its activity
I can't see any national security reason as to why information on the extent of the ash or volcanic gas should not be public. Show people the scientific evidence and they'll stop saying it's an over-reaction