Appreciating that it would be extremely politically unpopular, are there any policy reasons why a PM wouldn't just revoke the withdrawal, unilaterally cancelling Brexit?
The decision to revoke the Article 50 notification would have to be unequivocal and unconditional and it would have to follow the democratic process according to UK constitutional requirements.
Given the various Brexit laws, it might be interpreted as unconstitutional if the PM unilaterally revokes the notification. Lawyers and courts would have to rule on that, possibly both in the UK and in the EU.
Currently there seems to be a majority in the UK for a Brexit, just no majority for any one Brexit model. The legal situation would become rather muddled if they re-declare an Article 50 notification after this unequivocal and unconditional revocation.