UK/EU: Should airlines be allowed to decide weather to fly into the Icelandic Volcanic ash cloud?

I think there should be Xmg per cubic metre that any airline can safely operate in. Supposedly, ryanair proved today 4000mg is safe, but radar evidence suggests it may not be the case. Other airlines (BA to name one) are running test flights to see if their aircraft are affected. I think the airlines should certainly have a say in the matter!