As mentioned above, watch the euro emissions standards.
“Owners of vehicles that don’t meet the following emissions standards have to pay the ULEZ charge:
Cars: Euro 6 emission standards for diesels and Euro 4 emission standards for petrol-powered cars.
Petrol cars that meet the ULEZ standards are generally those registered with the DVLA after 2005, although cars that meet the standards have been available since 2001.
Diesel cars that meet the standards are generally those registered with the DVLA after September 2015.”