This is largely down to ozone depletion this last winter. The record strong polar vortex led to the worst ozone hole ever in this hemisphere. This despite the fact stratospheric ozone globally is starting to recover from its decades long decline.
We have a particularly holey bit over us at the moment, plus it’s clear and midsummer.
Pollution down, but levels have been picking up in recent days so I doubt that’s a big contributor. I can see an urban smog line over London this week for the first time in a while.
Brockley, South East London 30m asl