Sand under microscope
Volcanic dust under microscope
Volcanic dust is vastly more abrasive than sand. It has very sharp fresh edges and can be fine enough to penetrate very deeply. It can cause severe lung damage if inhaled.
Having clarified that, I'm with those who strongly suspect a degree of over-reaction here: for instance I can't help wonder how much risk there would be heading southbound out of LGW or LHR.