Just to defend the Met Office a little bit
"The output from the Met Office volcanic ash dispersion model has been compared with our neighbouring VAACs in Canada and France since the beginning of this incident and the results are consistent."
and
"The Met Office is the north-west European Volcanic Ash Advisory Centre with responsibility for issuing the Volcanic Ash Advisories for volcanoes erupting in this area in line with internationally agreed standards and processes. This means the Met Office’s role is to support NATS, CAA and other aviation authorities decision-making"
Maybe these standards and practises should change in the future, but that is how they are now.
"It is for the aviation industry and regulator to set thresholds for safe ash ingestion. Currently, world-wide advice from ICAO is based on engine and airframe manufacturers stating a zero tolerance to ash ingestion. This means that aircraft should not be exposed to any volcanic ash."
Nobody blames the ICAO so far. Looking at their website, they in fact seem quite pleased that everyone is following their guidelines.
On a separate note, I think the Eurocontrol area is different because the EU leaned on them to introduce a third "safer" zone. Not saying this is the wrong thing to do, but it is the reason I believe why their interpretation of the data look different.