Rov1100 -
I think you're overstating the threat to the UK.
The British Lung Foundation (http://www.lunguk.org/) have stated, “In light of the latest news that ash from the volcanic eruption in Iceland could reach the UK by Tuesday, we would advise people living with a lung condition in affected areas to carry their medication as a precaution....”. This statement is demonstrative of the perceived threat (ie. minimal).
By the time the ash cloud enters UK airspace, it is so diffuse that the threat to people with breathing problems is minute. Also, much of the ash cloud is at very high altitude and will not come to settle on the ground for many weeks/months and when it does, it will be over such a wide area (ie. the whole of Europe and western Asia) that concentrations of it will be almost undetectable. Unfortunately, the Icelanders who are in the immediate vicinity of the eruption are not so lucky and there's no doubt that for them the problem is very real and very serious.
However, from a UK public health perspective, I don't think there's any need to have concerns just yet.