"The bridge runs clockwise on the M25 and the tunnels anticlockwise..."
Just a point of pedrantry. The Datrford Crossing does not form part of the M25. The M25 is not a complete orbital route. It finishes just north and just south of the crossing. Roads that form the crossing are actually the A282.
Regular users may care to open a "Dart Charge" account. Crossings for those with an account attract one third discount (making it £1.67 for a car):
https://tfl.gov.uk/modes/driving/congestion-charge/dart-charge