There is some experience with these ash plumes shutting down jet engines. It definitely doesn't take much.

It might be possible to design a jet engine that could operate in that environment but that would probably require accepting a bunch of trade-offs to handle a rare situation, i.e. economy would be very bad.

I understand British SAR services are still open, but working on a case-by-case basis. I take that to mean they're only doing cases needing surface activities for now.