shirayuki,
is it possible that the same concentration of ash found in the sky around the volcano is the same one found near, i don't know, Heathrow airport for example?
Clouds as large as this one disperse and diffuse very slowly; especially in quiet weather. I would not be surprised if peak concentrations above the UK were 80% or so of when the cloud left the Icelandic coast.
It's been raining for the past two days and here and there i've read that if the wind changes and along comes the rain, everything will be cleared away and no more ash.
That's true for the lower atmosphere; however a large part of the ash is above the rain clouds.