A reminder to everybody that the London Low Emission Zone and the Ultra Low Emission Zone are both now live.
The Ultra Emission Low Zone can be seen at
https://tfl.gov.uk/modes/driving/ultra-low-emission-zone/ulez-where-and-when. I think, at present, it coincides with the Congestion Charge Zone but, unlike the Congestion Charge, it operates 24/7 and the Emission Charge must be paid on top of any Congestion Charge.
The Low Emission Zone can be seen at
https://tfl.gov.uk/modes/driving/low-emission-zone/about-the-lez. It also operates 24/7
Note that if you do not get off the M4 for the M25, getting off at the Heathrow Junction will incur the Emission Charge if your vehicle is not exempt - even if you just go round the roundabout and back. However, the roundabout at Junction 3 is not in the Low Emission Zone so, rather perversely, it will be cheaper to go on to Junction 3 and turn round if you have missed the M25 junction by mistake.
You can check if a UK registered vehicle will be charged or not at
https://tfl.gov.uk/modes/driving/check-your-vehicle-35896.
If your vehicle is foreign registered, you can register it with TFL if you think it should have an exemption but otherwise it will be assumed not to apply. This will, for example, apply to any visitors from St Benoit who drive here (although the last I heard, the French authorities were refusing UK requests for details of French owners if vehicles were caught by speed cameras).