Was nothing learned last year when they repeatedly closed in similar circumstances? And finally when something could be seen here (and felt in your face and mouth, everything was open.
As you can see by looking at the VAAC forecasts, ash at the surface doesn't mean there's ash at altitude. I'd imagine that ash has less of an effect if you're flying through ash for 2 minutes on the way up/down than if you're flying through the stuff for a prolonged time during cruise - although someone with more knowledge of ash effects upon jets can probably clarify that.
The eyeball is a rather poor sensor when it comes to particles too small to see
serving only the British Isles and Iceland, when Toulouse and Montreal could easily split this area between them?
Probably because the UK met office has better quality (and well validated) forecasts for that region of the world. Toulouse and Montreal don't - Iceland is likely to be outside the high accuracy area of their forecasting models.