The Advocate General for the European Court of Justice has issued his opinion on whether the UK can call off Brexit without the consent of other member states. His opinion reflects what Elaine Motion and the Petitioners have been arguing for. 
“In the light of the foregoing considerations, I propose that the Court of Justice should answer the question referred by the Court of Session, Inner House, First Division (Scotland) as follows:
When a Member State has notified the European Council of its intention to withdraw from the European Union, Article 50 of the Treaty on European Union allows the unilateral revocation of that notification, until such time as the withdrawal agreement is formally concluded, provided that the revocation has been decided upon in accordance with the Member State’s constitutional requirements, is formally notified to the European Council and does not involve an abusive practice.”
The full Opinion can be found here