It's fair to say that a good 25-33% of flights from this country that are heading southbound could still take place virtually risk free at a lower level. I'm not saying my numbers are correct but an example might be: A departure out of Gatwick could trod along 100 nm south at 6000ft before being out of the risk zone and then climbing higher and proceeding normally. Clearly this means temporary changes to airspace classification across borders which would affect GA but thats a lot better than having commercial traffic at a complete standstill for a month!
So you get airborne from Glasgow and head West climbing above FL200, then turn south. Somewhere around the Brest peninsula you lose an engine, but that's OK because you stabilise at FL220. Your options now are to fly back to Glasgow, press on to Santander or descend through volcanic ash on one engine. The dust cloud from SFC to FL200 covers most of France and South-West of the UK goes out to almost 20W.
I know that this will have a significant effect on airlines that are struggling, but the big hush would be much much worse.
Right, now off to get the BBQ ready for this afternoon.