I would also add in the period when the Eyjafjallajokull volcano in Iceland saw aircraft grounded too – that saw an unusually long run of blue skies too.
It might take some of the fun out of the discussion for the journalist, but by looking at VIIRs imagery for London over (say) the last 10 years you should be able to see what has happened in recent years as the measured upwelling radiance would need to be constant before you could safely attribute more stars to NO2 levels and the ULEZ.
Something like…
The website now actually offers the option for trend analysis on the light pollution levels. Impressive how dark some of the Motorways have become.
Perhaps upwelling blue light is scattered more strongly and never reaches space and so gives a false impression.
By comparison my home location has seen a small rise in light pollution as new house builds seem occupied by people afraid of the dark (despite every phone having a torch inbuilt) – even out here in the boonies where a crime is a major event!