I don't know that much about volcanoes, but there is very little information available to Joe Public which indicates that the ash cloud would be a danger.
Did you actually bother looking for any information?
I'm guessing no. Otherwise you would have found this page that details the basics (very basics) of how the London VAAC operates:
Met Office: Volcanic ash detection tool You would also have found this page, which shows the last known ash spread (top left) anf projected spread for the next few hours:
http://metoffice.com/aviation/vaac/d...1271354037.png
You'll notice that there is currently ash over London, even though the inability of our eyes to detect IR light means we can't see it.
By searching google scholar or a similar database you'd also find literally thousands of papers and articles on the subject of ash dispersion, ash detection, the effects of ash upon aircraft and countless related topics.
e.g.:
volcanic ash advisory - Google Scholar
What is so much worse about this one?
The fact that it's on top of your island rather than on top of some godforsaken pacific island.
(edit) And for those wanting proof: The theory and calculations behind volcanic ash detection are available through google scholar, although you may need to go to a library to gain access to the actual journal articles.
The data required for detection (AVHRR) is freely available from NASA. You can download it yourself, although unless you have a fancy satellite dish you will have to put up with a day or so delay from real-time.
Then you'll have all you need to calculate the ash spread yourself.