RR et al
My take is ICAO have been trying to tackle this issue for years on 2 fronts - (1) where is ash and provide warning for it (VAAC) and (2) what is the engine susceptibility level.
They got nowhere on (2) and then set a susceptibility threshold of 0 which the CAA, and NATS adopted to the letter. One particle in the whole airspace means close it!
It seems one of the culprits for this fiasco over the years is the engine manufacturers not supporting efforts on volcanos - plus the head of ICAO, CAA and BA not persuading them to. I don't understand how in PPRUNE and the wider press, RR et al have so far got away scott free.