I recommend Simonpro's replies. I'm neither an atmospheric scientist, nor an engine engineer, but I think there is definitely cause to worry about flying through a spread-out dust cloud, as well as a BA9/KLM867-style eruption plume.
Any rock-like material will melt onto hot turbine parts, which are some of the hottest and most carefully-shaped surfaces that are engineered. If a random coating of volcanic glass improved jet engine performance, one would have been fitted in the factory. Piston engine/turbocharger surfaces are much cooler, and so I doubt they would suffer.
No rock ash is good, but there is a background level, definitely from meteorites and averaged volcanic emissions, and perhaps from wind-uplifted sediment, with which aircraft engines cope all the time.
Meteorite particles rain down at the rate of a few hundred thousand tons per year, over the whole earth (
Access : Space Density of Dust in the Stratosphere : Nature 4838 (1962) p. 269). This gives a hard lower limit to the amount of meltable particles in the stratosphere.
Iceland currently seems to be chucking up a few tons of ash per second into the stratosphere - a rate that is about a thousand times higher than from the meteorites, and concentrated geographically (
Satellites Providing Rapid Estimates Of Iceland Volcano Emissions - Science News - redOrbit).
Volcanos that went off during the last few years also contribute to the background level of stratospheric ash. I can't immediately find a number for this, but individual volcanos seem to make a factor <10 difference to lidar measurements of particle density in the stratosphere, so it's probably not hugely greater.
With a few volcanos a year throwing ash up for a few days each, and a couple of years needed for it to rain back down, that would imply something like an average of one-to-ten million tons of volcanic ash in the stratosphere at any time, likely a factor of 10 more than from meteorites, especially at aircraft heights in the lower stratosphere.
A diffuse, drifting cloud of volcanic ash from Iceland does seem to be a real risk. It is reasonable to expect levels of turbine-clogging ash over the UK to be up by a factor of at least a hundred from the background at the moment. Where a safe line can be drawn, I don't know, but Eurocontrol seems to be confident that it's below where we are now.