Brother Hesekiel
Nobody knows what will happen in a case that has never happened before in history. However, it's fair to assume that those who are legally residing in the UK pre-Brexit will be allowed to remain. Working against this would most certainly cause other EU countries to expel Brits who reside there.
But it wouldn't hurt to naturalize in the UK once one becomes eligible.
Spock (rhp)
this is the subject matter of the negotiations that haven't even started yet