The British Met Office took a wild guess at the ash cloud and its dispersal in the absence of any hard data. Data was unavailable because the Met Office research aircraft was being repainted.
The Quazi Government Organizations (Quangos) that provided air control and civil aviation safety quietly fell into line behind the Met Office wild guess which proved to be very wrong. Not surprising when the head of the CAA was chosen for a total lack of knowledge about aircraft.
The chaos was ended only when British and German airlines decided to send up their own research flights to find the Met Office claims were wildly inaccurate.
It seems the old habits of climate fraudsters die hard – its so much easier to come up with a conclusion and then try to fiddle the data to fit the incorrect assertion.