I have "pinched" this from a weather forum. It explains much:
If the non-volcanic cloud layers disperse or have large enough gaps, you should easily be able to see these volcanic ash layers. They will look like dirty cirrus or more properly cirro-stratus, and they should be drifting south at the height they are currently at (about 15-20k ft). They are not white like normal cirrus or even grey so much as a sort of light beige to brown colour, but they do start to resemble cirro-stratus ice crystal clouds from what I recall of similar intensity volcanic ash clouds here in 2008 (from Alaska). They won't block the sunlight by more than 5-10% and you may see parhelia (sundogs) associated. As to where and when I am gathering the more concentrated parts are over northern Scotland now but more dispersed early segments are probably visible as far south as central England.
I expect this will get worse in terms of disruption and better in terms of viewability because it will take probably at least 3-5 days for this initial burst to disperse even if the volcano stops erupting shortly, which of course is not a given -- and the weather pattern favours more direct delivery of the ash plume to the British Isles as time passes now to Sunday. However by Sunday with that cold front coming through, the skies may become too cloudy to see the volcanic ash, yet at the same time the more turbulent conditions may bring some concentrations down towards the surface and have an impact, possibly moderate (I would doubt severe) on air quality or health considerations for the vulnerable.
Just to give you an idea, when Mount St Helens blew in 1980, ash fell to a thickness of a millimetre at a distance downwind about equivalent to northwest Scotland in this case (it fell to much larger depths closer to the volcano but by analogy those locations would be over the ocean except for the Faeroes which could conceivably see a half inch ash fall from a major Icelandic eruption).
I have the feeling this might cause disruption for over a week, it's bad luck that the wind direction is NW at upper levels for that whole period, more or less.