Getting the Congestion Charge "cancelled" is unnecessarily complicated IMHO.
What I want: Drive into London, Congestion Charge camera does recognition on Number Plate, checks DVLA computer and finds I'm in an electric vehicle.
What actually happens: Request form, fill in form, send evidence that the vehicle is electric, pay an annual fee (£tenner I think), wait at least a couple of weeks for their confirmation reply ... then allowed to drive into London.
In my case I've paid two years in a row and not actually driven into London yet (I do, but only "from time to time").