EI Premier, the dispersion model used by the London VAAC has the information (what there is) provided by the Icelandic Met Office as its starting point. From there, NWP models forecast the movement of the cloud.
But, it's a FORECAST using the best model available. Unfortunately, meteorological time machines do not yet exist for someone to pop over to tomorrow or the next day to see where the ash is; nor do governments, airlines, nor the taxpayer want to shell out more money to make the model and observational data better than currently available.
Yet again, the Met Office/VAAC merely fulfil the brief given them by ICAO to FORECAST the movement of ash. It's up the Aviation Authorities to judge wether airspace should be shut.
As pointed out earlier, FAAM will be sending up a research flight later to find out how much (if any) ash is up there. Until then, there's no point getting one's knickers in a twist.
Perhaps, instead of asking so many questions, you could suggest some possible solutions?