Brexiteer
And there is a word.
A new word about it
That word is
Brexiteer (i.e. from 'Brexit' + '-eer'). Pronunciation: /brɛksɪˈtɪə/. It's a new English word. It means a person who is in favour of the United Kingdom (UK) withdrawing from the European Union (EU).
I learned well about it from
https://en.oxforddictionaries.com/definition/Brexiteer.