NO2 is mostly generated from burning fuel, that's why in most of the countries in the map you only see yellow blobs over major cities due to higher car traffic, and almost no NO2 in rural areas.
The major blob over central europe and south uk makes no sense and I don't think that excessive farming can justify it.
If you look at a worldwide map of NO2 emissions [1] you can see that besides that large area in Europe only China has concentrations of NO2 that high.
[1] https://so2.gsfc.nasa.gov/no2/pix/regionals/scripts/sliderav...