Pollen could definitely be a key part of it, but I doubt it's the whole story. Checking the PM 2.5 count for yesterday for the Bloomsbury sensor in central London, it's actually about 70% of that at any of the sensors in the East Bay (Berkeley, Oakland, etc) or San Francisco.

http://www.baaqmd.gov/about-air-quality/current-air-quality/...

You need to convert units, because the US readings are given on the AQI scale, whereas the London ones are raw measurements:

https://aqicn.org/calculator/