We've actually left the EU already.

"Whichever way one looks at it, the Agreement was either unlawful or made for an unlawful purpose or ultra vires.  That means that the UK left the EU on the 29th March 2019 by default as there was no valid or lawful impediment to prevent it."   

So says Stanley Brodie QC in an article.  He recently retired from practice at the Bar of England and Wales after over 60 years, including over 40 years as Queen’s Counsel.  He practised at the commercial Bar from Blackstone Chambers, being instructed in a wide spectrum of cases. He is also part of Lawyers For Britain.   Quite a smart chap then.   

So bearing in mind his legal view, perhaps the next Conservative Prime Minister could find it useful to say on Day One of his or her time in office that, since we have already left the EU legally, all we can do is negotiate our relationship for the future.