Changes are being made to improve the air quality within Greater London through the upcoming expansions of the existing Central London Ultra Low Emission Zone (ULEZ) to create a larger zone up to, but not including the North Circular Road (A406) and South Circular Road (A205) from the 25th October 2021.
An expanded boundary description can be found here: https://tfl.gov.uk/modes/driving/ultra-low-emission-zone/ulez-expansion-boundary