One of the longest (the longest?) set of continuous sunshine data for England is from Bradford, which conveniently used to suffer from horrendous pollution until a few decades ago. It's a very dull city anyway, with a 1981-2010 average of 1265 hours, but while I've never taken the effort to work out old 30-year averages from the dataset
here, I have added up the monthly figures for 1912 and got just 816 sunshine hours, whereas I would be surprised if any year in modern times has gone much below 1100.
Having a look at how often sub-20 and sub-10 hour months happened during the 1910s and 1920s compared to today, I can imagine the pollution having taken up to 50% off the long-term average 'natural' amount of winter sunshine the area would have got otherwise. I'd also be interested to know what London's December 1890 with zero sunshine would have been if not for pollution, as anything below 30 hours counts as very dull down there nowadays.