The article 50 is clearly stated in that regard. UK and EU can extend the negation but the outcome is clear.
I don't actually think this is true. Article 50 is not at all clear. Leaving is triggered by either the date of entry to force on a withdrawal agreement, or two years after a notification of intent to leave is issued. However, it doesn't specify whether a notification of intent to leave is still valid if the notifier no longer has intent to leave - this is very unclear.
If anything, legal opinion weighs in the opposite direction. Lord Kerr, who drafted the treaty and might be cited as an example of the intent behind it, argues that the notification requires intent to remain valid. Professor Enchelmaier of Oxford's European Law department concurs, as do most legal minds I could cite - it seems to be the majority opinion.
What's more, the EU also abides by the Vienna Convention, which is a sort of ur-international treaty governing what a valid international treaty ought to looks like, which notes that all parties have the right to withdraw from an international agreement (the notification being a form of this) given significant and relevant change of circumstance, which is fairly loose, but 'not wanting to withdraw' would seem both significant and relevant.
Either way, the case would almost certainly be challenged in the courts by whichever side it was decided against, and would end up before the CJEU, but from at least from my layman's perspective, enough experts seem to think they way in favour of revocability that I'd feel somewhat confident in it.