There has been considerable discussion in the UK about whether or not the country must leave the EU having signalled its intention to do so by issuing notice under Article 50 of the Treaty of Lisbon.
During the court action which forced the UK government to abandon plans to issue the A50 notice under “Royal Prerogative”, no determination was made as to the revocability of notification, but the governments lawyers argued that it would not be withdrawn and should be treated as such. The plaintiff in the case was not arguing about this issue, but rather if the government had authority to embark on a course of action which would result in a change to the rights of UK citizens without recourse to parliament (they hadn’t).
The decision to withdraw from the EU is a sovereign decision of the UK government. It has always seemed odd that people should believe that having taken the step that there was no way back. Some believe that the decision cannot be reversed; others that reversal would require the consent of either the European Court of Justice and/or the approval of the other 27 nation states of the EU. If this were true, it would imply that the EU27 have the authority to determine the future for a sovereign nation which is part of the bloc - this must be nonsense. Ironically, Brexiteers have demanded a return of sovereignty from the EU, but that was something the UK (or other EU states) never lost; a point made in the preamble to the Article 50 Bill which had to be passed before the UK Prime Minister had the right to issue the notice. A further test of the folly of the position of irrevocability is that it would imply that should the UK elect a proEU government on a mandate to remain in the EU before the end of the notice period (29/3/19) that that government would be bound (effectively) by the decisions of the current administration (itself a successor/continuation of the government which issued the original notice).
Legal action is underway in Scotland which seeks to gain clarification, ultimately from the ECJ on the subject of the revocability of the A50 notice), however, a statement from Donald Tusk, President of the Council of Europe made yesterday to MEPs in Strasbourg surely puts the matter to rest:
"The hardest work is still ahead of us, and time is limited. If the UK government sticks to its decision to leave, Brexit will become a reality - with all its negative consequences - in March next year. Unless there is a change of heart among our British friends.”
“Wasn't it David Davis himself who said 'if a democracy cannot change its mind, it ceases to be a democracy? We, here on the continent, haven't had a change of heart. Our hearts are still open to you."
Therefore, it seems that there is no legal impediment from the EU side for the UK to end the Brexit process if it so wishes. The question is only does the UK wish to carry Brexit through if a “bespoke arrangement” and a “deep and special relationship” outside the EU prove impossible to attain?