I'm not sure, but I was curious as well. I have a 2011 X5 3.5d that has SCR and uses DEF fluid. I googled around a bit, wondering if pm2.5 was reduced using DEF, or whether the DEF was for other pollutants. I'm not sure how clean it is relative to gasoline engines in terms of particulate matter, but obviously it's better than without it.

Of course, much of the fleet in London will be older. It was shocking to me to see (in the article) how diesel vehicles had gone from 1.x million in the 90s to 12+million today.