I found some more to add to the mix earlier today (thought I'd add it here as these arguments come up time and time again), which is less wishy-washy than I was above.
Jean-Claude Piris is, to quote his Twitter bio, "French. Consultant EU law and International law. Former Director General of the EU Council’s Legal Service from 1988 to 2010" (he's been a lot more, but it's a bit of a mouthful, so see below*):
Steve Bullock @GuitarMoog
I've always argued that withdrawing Art50 notification successfully was (unusually) primarily a political rather than a legal question 5/
JC Piris @piris_jc
But it is legally possible to!nobody could expel the U.K. if it decided to remain according to its constitutional practice before 29th March 2019. Unilateral choice. The 27 cannot impose any condition
Juho Romakkaniemi, current Head of Cabinet for the Vice-President of the European Commission, chimed in:
Juho Romakkaniemi @Romakka
Of course. If Britain decides and notifies before March 31st 2019 that it does not want to leave, that’s enough: #Brexit van be cancelled by a single letter. It’s in the end of the day a political, not a judical decision.
…
* "Jean-Claude Piris served as the Legal Counsel of the Council of the EU and Director General of its Legal Service from 1988 to 2010. He is an Honorary French Conseiller d'Etat, a former diplomat at the UN and the former Director of Legal Affairs of the OCED. He was the Legal Advisor of the successive intergovernmental conferences which negotiated and adopted the treaties of Maastricht, Amsterdam and Nice, the Constitutional Treaty and, finally, the Lisbon Treaty. He was also Senior Emile Noel Fellow and Straus Institute Fellow at New York University."
= new reply since forum marked as read