So the new CAA rules are: you can fly if the ash concentration is less than 0.002g (2 milligrams) per cubic metre of air.
Fortunately the ash cloud above the UK only has a concentration of 0.001gm-3 so that's alright, all UK airports are now open.
There's some suggestion of higher density "dark blobs". They'll just have to fly around those.
The only question remaining is: did we need to stop flying at all?