As someone said above...
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Clearly, it is vital to know the relative constituents of a plume and, more importantly, their concentrations over the area under consideration. If it is largely grit and silcon affecting the UK, which I suspect, then there should be no impediment to re-starting operations. Operations are routinely carried out in much worse conditions world-wide in, for example, the middle east when sand storms can reduce visibilty to 600 m. -a "contaminant" concentration hundreds of time worse than prevails over Europe today.
If there are high levels of soot in the sample over the area in question then urgent research needs to be carried out by the engine manufacturers to agree a protocol with regulators and governments.
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Someone who thinks we are dealing with "grit", "silicon" and/or "soot" doesn't inspire much confidence as an opinion former.