Whose idea was it to exempt low-emission vehicles?
The congestion charge is meant to reduce congestion. Exempting vehicles because they're "cleaner" never made sense.
The "pollution charge" is called petrol tax, and it's a separate thing.
Hint to Sadiq Khan: parking spaces. It's within your power to divide up every road in London into areas where (a) you can't park at all, or (b) are divided into discrete, marked spaces, each owned by an identifiable entity. At that point you can tax them.