The real scandal in this (and it is rapidly becoming one) is that despite decades of jet operations in (on a couple of nasty occasions) and around volcanic activity, there has still been no real research into what is and what is not an acceptable level of contamination.
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.
The fact is that operations continue in, for example, severe icing conditions, wind shear, torrential rain, snow covered runways and dust storms perfectly routinely because the risks have been assessed and mitigated, not removed, which is impossible. It is a disgrace that this has not been done on this occasion.
It is time for the profesionals to deal with this, not administrators or government agencies, who have neither the knowledge nor the will to make a proper, objective assessment.