Volcanic dust.
10 or 20 years ago, when we still had a "met" flight at Farnboro, there would have been a rush to; a/ map the cloud and
b/ leap into the dust to check effects and measure the crap.
Any engine problem could be overcome with a touch of the deadsticks. Since the RAF don't seem to be interested, the safest alternative would seem to be launching a 4-engine machine and minimising the possibility of an all-engine-out by having all engines at differing power settings (one at idle) and trying some suck & see.