The Enviro 500EV isn't currently
marketed in the UK, but that doesn't mean that it won't ever be built for the UK. There are some diesel E500s operating in the UK, after all, when they were largely intended for the far-east markets.
Just because a particular model isn't on the relevant section of the Alexander Dennis website, it doesn't mean that it will never be built for a different market. If Lothian (or any other operator) wanted to order E500EVs, it would probably happen as long as the order is big enough - money talks!