Here are some extracts from TfL’s ULEZ pages:
To help improve air quality, an Ultra Low Emission Zone (ULEZ) will be in place in central London from 8 April 2019. Most vehicles including cars and vans will need to meet new, tighter exhaust emission standards (ULEZ standards) or pay a daily charge to travel within the area of the ULEZ.
Charges for cars (if ULEZ standards NOT met):
Daily charge – ULEZ (Low): £12.50
Penalty charge: £160 (reduced to £80 if paid within 14 days)
From 25 October 2021, the ULEZ area will be expanded to include the inner London area bounded by the North and South Circular Roads.
Worth noting that the North and South Circular Roads are themselves outside the zone.