The UK Prime Minister, Boris Johnson, has clearly set out his position in that he wishes to leave the EU on 31st October with or without a deal. I.e if further negotiations with the EU do not result in an agreement which can be approved by Parliament, the UK simply leaves anyway.
Those opposed to a no-deal Brexit are discussing possibilities to prevent leaving without a deal. For example, it has been suggested that an act of Parliament be proposed; discussed; agreed by the Lords; and signed into law, requiring the government to request an extension beyond 31st October, if no deal has been reached by then. This is a possibility, given the amount of opposition there is to leaving without a deal in both chambers.
So - let's suppose this came about.
As I understand it, international dealings must be handled by the government: i.e the executive, not the legislature. Therefore as per the separation of powers, the legislature (Parliament) has no authority to request an extension by itself. It can only legislate to require the executive to do so. If my understanding is wrong, can someone correct me?
So what I want to know is - let's suppose that the legislature demanded that the executive requested an extension from the EU - but the Prime Minister ignored it, and did not make that request. Let's also suppose that it only became apparent that he did not make that request, late in the evening of 31st October.
What would the legal position be? What would be the consequences to the government of ignoring a law which had just been passed? Is there historic precedent involving any similar situation?
Note: I understand that Brexit is a controversial topic, and I am not asking about no-confidence votes, elections, proroguing, or anything else. I'm simply wondering about the above.