Fuck, that is some question. I really don't know. He would be breaking the law and I suppose he could be prosecuted and if convicted that would bar him from office.
Hopefully Parliament would have the sense to get rid of him, by no confidence or maybe it might even fall to the Queen to dismiss him on the advice of the Privy Council. It would certainly be unparliamentary as he would have effectively seized power from Parliament and in all effects would be an elective dictator.
I posted a week ago or so I was worried we were descending into fascism, if Johnson did do that, the descent would be complete as it would be a coup by the executive against the legislature.