No idea what the point of them was/is though. Probably cost a fortune to implement for zero gain.
Why did the government introduce green number plates?’
When green number plates were announced, the-then Transport Minister Rachel Maclean said they would “raise awareness” of electric cars. This awareness raising would potentially encourage people to buy an electric car ahead of the planned ban on the sales of new petrol and diesel cars in 2035.The Department for Transport also said green plates could “unlock a number of incentives for drivers”. Precisely what these would comprise was not stated, though it was hoped, for example, that councils might offer reduced parking rates of EVs with green plates.
These incentives have not to materialised for a number of reasons, including how people fitting green plates to petrol and diesel cars could be prevented from fraudulently obtaining any benefits that might be offered, and the fact that if any incentives were wanted to be offered for EV drivers these could already by facilitated due to the fact that what fuel a car is powered by is already recorded, with this information being obtainable to organistions via the Driver and Vehicle Licensing Agency already.
https://www.carwow.co.uk/blog/green-number-plates-explained#gref