https://publiclawforeveryone.com/2017/10/16/can-parliament-block-a-no-deal-brexit/
"Parliament can say whatever it wants in a Bill. The European Union (Withdrawal) Bill could thus, for instance, be amended to provide that MPs must be given an opportunity to vote on any deal agreed between the UK and EU governments. It could even stipulate that the UK is not to exit the EU unless there is a deal that MPs consider acceptable. But such statutory provisions would amount to nothing more than the legislative equivalent of the instructions issued by King Canute to the incoming tide. For just as Canute could not control the laws of nature, so the UK Parliament’s sovereign capacity to make domestic law cannot be used to control the operation of EU law. The EU law in question is Article 50(3) of the Treaty on European Union. It provides that:
The [EU] Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period."