Funnily enough, it seems as though the closure of UK airspace is partly based on one model provided by the Met Office that is based on little actual data from the eruption. It seems to be based on the results of various Lidar readings (which tell them there is ash, but not how dense it is) and one 4 hour test flight that produced inconclusive results.
http://www.theregister.co.uk/2010/04...volcano_model/
Appatently, they haven't even sent up any Weather balloons, which could have given accurate data.