Theresa May does not want to leave the Customs Union or Thatcher's Single Market.
She knows perfectly well that any final deal on Brexit would require parliamentary approval in order to enact the legislation repealing and superseding the European Communities Act.
And she has absolutely no intention of trying to deport EU nationals who were already here, a move that would be massively unpopular if any attempt were ever made to give it practical effect.
But then, she does not want Brexit at all, and her party made her Leader in that full knowledge.
No one who did want Brexit was even able to contest a Leadership Election against her.