Have you read the CAA FCOM that you link to?
The volcanic ash cloud over Europe created an unprecedented situation for aviation. At no other time has such an extensive amount of volcanic ash settled over some of the most congested airspace in the world and in stagnant weather conditions. The existing guidance for 'planes encountering volcanic ash is to AVOID AVOID AVOID so allowing for no interaction between jet engines and ash. Because this situation had never occurred before, the international guidance was largely untested and was never intended to cause such widespread disruption. It assumed there would continue to be flight paths which would allow avoidance but in this case there was not the space to do so.
The CAA has led the international discussions and investigation which has enabled manufacturers to come to an accurate and evidence based set of restrictions and agree engine tolerance levels to ash density. When you are dealing with people's lives it is not enough to just make up a less restrictive standard, you have to agree one based on robust scientific evidence and data. The UK 's work to prove this has now been adopted across Europe.
This talks of the interaction between jet engines and the VA, as jet engnes have vast amouts of unfiltered air passing through them blocking on the turbine cooling ducts is posable.
Piston engines use far less air and this air is filterd, in short these are two different types of engine and the technical part of the CAA knows this as is indicated by the FCOM. after all if this VA was such a danger to piston engines then the motorways would be clogged with broken down cars!
The problem is that a bunch of people who don't understand the the difference between piston and turbine engines have made a "blanket" rule by those who have over reacted once and can't get themselfs out of this mess without loosing face.
Its just the sort of thinking that had the CAA doing GPS approach research to approve a system that had been in use for more than ten years in the USA just because they had dug themselfs into a hole by ignoring the GPS system for years.
As an engineer I have been taking notice of another CAA document that recomends regular inspection of engine air filters, these extra inspections have yet to find anything unusual.