Here's an interesting take on the Article 50 case that's being heard in the Supreme Court.http://www.prospectmagazine.co.uk/politics/supreme-court-government-brexit-appeal-article-50-revocable
The author, a QC, thinks that the important question to be answered isn't whether or not Article 50 can be invoked by Royal Prerogative or not, but whether the notice can be revoked once it's being issued.
If it can't be revoked, then issuing the notice of an intent to leave means that you're out regardless. It gives Parliament very little choice in whether to accept a deal on its own merits, as it'll be take it or leave it.
Conversely, if it is reversible, then Parliament says, "Actually, we think that sucks, and we'd rather stay."
My suspicion is that it would be revocable in practice, the EU is good at creatively interpreting its own rules, but it might not necessarily be in strictly legal terms. The wording of Article 50 doesn't include anything about cancelling it.