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I don't think the data has been published properly, as far as I can tell. A article [1] by York University itself is clearer on what they are saying. Look at the second figure here and note

> In the graphs below, the blue bars show the average level of pollution from late February through to late March, taken from 2015 and 2019. The red bars show the average level of pollution between the same dates this year.

You can easily see that PM2.5 has roughly halved in London.

[1] https://ncas.ac.uk/en/18-news/3057-air-pollution-falling-acr...

I realise this anecdotal, but as a Londoner living in Zone2, I can actually see and feel the difference in air quality. The skyline is far clearer on average, and I definitely notice myself breathing easier, and I do not have asthma or any other respiratory or even allergy issues.

It's badly presented, but it seems to show the difference of the current values to two different baselines (earlier this year, and a long-term average).