As for the validity / accuracy of the Met Office model, IIRC, other VAAC were requesting access to it after Eyjafjallajökull as it was significantly more accurate than anything they had. However the fact is that forecasting is an inexact science, forecasts will change, and they're based on the best data available at the time of issue, which may not necessarily reflect the actual observations at the time of validity. In other words, forecast does not equal observation. Thats true the world over. In the case of ash, I would imagine it has to show worst case possible spread and concentration - in some places there will be less, in others more.
Of course, however:
I think that perhaps the industry would be better served right now with dual use of real time analysis and NWP modelling, as opposed to ridigly adhering to the NWP modelling. A mix of both will yield the best outcome, whilst still maintaining safety for all.
Safety trumps all other factors, but it is equally important to ensure that when viable - flight operations should continue.