Yes, from 1 Jan 2018 all new registered taxis are required to be all-electric or petrol-electric hybrids. There's going to be a £5k grant for taxis over 10 years old (note there is a maximum age of 15 years that has long been in force), which will hopefully push the oldest third of taxis to get replaced more quickly than normally.
That said, the UK is committed to eradicating all petrol and diesel cars over the next 22 years, and the black cab drivers are looking at what you're talking about: https://www.theguardian.com/business/2017/jul/11/black-cab-t...