@c2clo. Taken from the website. I have been to London a couple of times earlier in the year and have not had to pay.
You don’t have to pay the Congestion Charge or register with us if the vehicle is recorded at the DVLA in one of these categories: Two-wheeled motorbikes (and sidecars) and mopeds. Vehicles used by disabled people that are exempt from vehicle tax and have a ‘disabled’ taxation class. …
Disabled motorists may cross the Dartford Crossing without charge if they are travelling in a vehicle which is exempt from Vehicle Excise Duty (VED).