The Prime Minister would have had to propose a motion to agree to prorogation. A debate would have ensued, and members would have discussed the issue. Had the motion been adopted, it would have confirmed that the Prime Minister had the confidence of the House and there would have been no questions asked. All the partisan debate could have taken place.
Had the motion been defeated, it would have meant that the Prime Minister could not go to the governor general with the request. Were he to have done it anyway, it would have been assumed that he did not have the confidence of the House.