I think the mayor has a different agenda and it ain't the environment. I think his plan is to banish Uber, then discourage 'polluting' vehicles from London. Making it easier for him to force all the black taxi drivers to go 100% electric by 2020. I think he wants to leave that as his legacy, "I made all London public transport electric". That's my theory.
Arguable the additional runway at Heathrow will reduce pollution as the planes won't need to stack and circle London so much, have a look at one of those live flight maps. Most planes fly 3or4 circles over london before landing. Also as part of their bid, Heathrow promised to make every single vehicle(2000 vehicles) on the airport electric.
I wonder where the mayor thinks electricity comes from?... Local generation! We'll just have to burn more gas and have more incinerators to generate the electricity... Surely that defeats the object!