In order to make Brexit happen, the Prime Minister is going to have to ask the Queen to sign a law repealing the European Communities Act (1972). But to ask her to do that is tantamount to asking her to risk signing away the Union (because it will precipitate a Scottish Referendum).
No Prime Minister would ever put the Queen in such a constitutionally awkward position.
So Theresa May instead will call an election (though with the Fixed Term Act it’s a bit more complex than before, but the other Parties will cooperate apart from the one UKIP MP) on the single issue “Brexit or Save The UK?”.
And of course she will win it, with most Leave voters responding to the patriotic ‘Save The Union’ message.
(This explains why Theresa May has not tried to call an election yet, and will not do so for some time) .