I think it’s perfectly possible to have a career in Clojure in London. But, of course, I couldn’t advise anyone to move to the U.K. until the final state of Brexit is known.
Don't choose a language because it will make you money. Choose language because it's something to learn and you'll enjoy programming in it. You can know more than 1 language. The main one is that which will earn you an income. The other, or others it to play and enjoy what the language and environment has too offer.
There are a tonne of Elixir jobs in London at the moment. It's a great city to live and work in.
Where exactly? I don't see many. Indeed.co.uk has only a handful of Elixir jobs by title search and Angel.io has hardly any.
Look on the elixir job boards, slack and forum. Some companies are hiring for multiple roles. E.g Whitehat are hiring 10 more devs, Toyota are building a team of 20. Etc. There are more open positions than Elixir devs in London atm.
Even in London there are only a handful of Clojure jobs. Clojure growth has stagnated. Likewise with Elixir though Elixir has a better chance in the long run due to the community's support for Phoenix.
Worst case Elixir result is you become an Erlang programmer instead.