As usual, the BBC page is showing LOW for the forecast today.

The Defra London monitors page, reflecting past performance shows low
for ozone,sulphur dioxide and NO2, BUT

for PM 2.5 particles , 1 moderate (6) and 7 high (2x7,4x8,1x9)
for PM 10 particles 3 moderate (1x5,2x6) and 4 high (3x7,1x9)