If it is indeed the case that UK is NOT flying data-collection flights round the clock in areas where ash-particle concentrations are considered 'marginal', then the first question is: Why not??!!
It's already been suggested that there are a number of scrapper Nimrods not doing anything although presumably (to some extent?) serviceable. Use them and then UK Goverment might do something useful AND get back some of the collossal previous investment in these piles of superannuated junk. We might even strike lucky and find that the MR variants still have air-sampling kit fitted from COld War days....
Even if other types had to be used, at the current rate of financial loss by UK Ltd and especially air transport, scrapping ONE AIRCRAFT EACH AND EVERY DAY after 'unsuccessful' data collection would be well worthwhile.
So far, based on reports here, the Dutch have flown THROUGH rather then 'IN' the dust for short periods, and other European carriers have done some point/point flights which either avoided the dust via height selected or chosen route. Who's ACTUALLY, RIGHT NOW flying at heights / routes where 'acceptable' particle densities are believed to be, with continuous sampling of the dust? Apparently no-one. How silly is that?