[apillar]

It's my understanding once Article 50 is triggered, it can't be untriggered, so at this point even if you had some leftist court in the UK try to reverse the decision or order a new referendum that reversed the results from the Brexit vote, it would be no easy task to get the UK back into the EU. Essentially they would have to start from scratch like a new country asking to join. So this really is a huge step.

[naturalman1975]

Technically, it seems that if all 27 EU countries (the UK and the remaining 26) unanimously agreed over the next two years to cancel Brexit, it could still be stopped. That is incredibly unlikely but it is technically possible. The process can no longer be stopped by the UK alone.