I was about to post that I thought VMC arrivals and departures not above 5,000 QNH might be a good idea with IFR plans being activated (or deactivated) outside the affected area.
Then I saw the pix of the crud that is beginning to settle on parked cars here and there across the UK and had second thoughts.
I would also venture a guess that modern ATC systems are very reliant on computer control. I bet they're not programed to cope with major changes to routine like low-level VFR departures and arrivals? Which might explain the sudden and complete shut-down when people are reporting clear skies in their neck of the woods.