A new European Union website devoted to Article 50 negotiations makes it clear that there is a way in which the Brexit notice could be revoked.
The UK cannot unilaterally withdraw the letter sent by Theresa May, but it could ask the EU to agree to revoke the notice.
The new Article 50 taskforce website (part of a larger new site) has a Q&A page including this:
Once triggered, can Article 50 be revoked?
It is up to the United Kingdom to trigger Article 50. But once triggered, it cannot be unilaterally reversed. Notification is a point of no return. Article 50 does not provide for the unilateral withdrawal of notification.
In other words, the UK cannot simply say, “Oops! We made a made a mistake. We didn’t mean it. We are staying in the UK.” But the UK could say, “It has become clear that leaving the UK is not good for our country, not is it good for the other 27 members of the UK. It would be best for everyone in we could withdraw out Article 50 notice and find a way to work together to our mutual benefit.”
Obviously, that is not an option that Michel Barnier, the chief negotiator, can put forward, but several leading people in the EU including Guy Verhofstadt, lead negotiator for the European Parliament, have suggested that one day the UK would rejoin. Verhofstadt wrote in the New Statesman:
I am also sure that – one day or another – there will be a young man or woman who will try again, who will lead Britain into the European family once again. A young generation that will see Brexit for what it really is: a catfight in the Conservative party that got out of hand, a loss of time, a waste of energy, a stupidity.
That process of rejoining would be long and painful. But as the disadvantages of leaving will become clear during the coming 18 months of negotiation. The failure of the Leave campaign to deliver it promises, the loss of jobs, reducing wealth, the threat to the United Kingdom, difficulty of forging new relationships outside the common market, the loss of influence in the word and the need for Europe to hang together in an unpredictable world will all become clearer.
Far better to swallow pride before we are outside the tent.