Does greater London not fall under his jurisdiction? I believe he is what's known as a metro mayor. The devolved system was setup in the 2010's and include places like Manchester and Liverpool. It gives the mayor's office devolved powers from the government to run multiple local authorities as one big authority.
So I think, if I have got this right, the government gave the London mayor the devolved governmental powers to make these decisions, the councils are controlled by the mayor's office, so they couldn't argue or stop him and finally the public voting for him made him democratically chosen and able to exercise the mayor's power.
If I'm right, him extending the ULEZ zone, which was set up by Boris Johnson, I think, is well within his powers as mayor and because he has governmental devolved powers, there isnt much the government can do.
Short of a revolution, I can't see a way out of it. We should be more European and set cars on fire, block roads with tractors rather than mumbling and groaning in true British style and then paying the £12.50 anyway...
Viva la revolution...