I have since read in the New Scientist (June 9th) that reported that the electric car noise law will become mandatory on all new cars sold in the EU in 2020 (or was it 2021?).

Because of the multifarious regulations re cars in many different countries, most cars have all the various requirements where they don't conflict. So regardless of the Brexshit mess, they will probably have the noises in the UK. I do seriously expect there to be custom retro-fit kits and personalised noises. And bloody volume!