@Hossenfeffer Barcelona has one. We drove a far from new SLK into there. (We don't live in London, either)
It required registration with an automatic system, in English, and a e5 fee.
Nuisance, but it is now recognised as compliant not just in Barcelona, but throughout Spain.

That's an area of nuisance in England, that this is multiplying effort, and an incompetent and uncaring central gov isn't coordinating it.