1. Doing nothing
2. Snap parliamentary election leading to a May loss
3. New referendum
4. Ireland/Scotland make it too complicated
5. Europe makes it impossible
Let's take a quick look at each.
1. Doing nothing: Article 50 of the European Union agreement has specific rules for how members can voluntarily exit the EU. The U.K. will lose the membership in both the common union and its customs agreement; a negotiated set of replacement treaties and rules would be proposed, which then would require ratification by both the EU and the U.K.
The key economic aspect is that all of the advantages of the EU treaties covering trade relations among members would be replaced by less-favorable covenants. How much of a disadvantage this amounts to is the subject of debate between all concerned.
The bottom line it that whether the Brits get a full withdrawal agreement, or only a temporary transitional agreement, it is likely to be much less friendly to the U.K. economy then staying in the EU.
2. Snap election: What else could derail Brexit? How about more Cabinet members supporting staying? Then there is the issue of May's popularity: a year ago she was considered a dead woman walking. Her support is recovering among her fellow Tories, but her polling is still underwater and an electoral loss would amount to a repudiation of whatever it is she thinks she's accomplishing.
3. New referendum: The British public has learned much since the initial vote. About half of Brexit voters have been supportive of a second referendum (you can guess which half), amid a chorus of calls for another vote on Brexit.
A steady drumbeat of media reports revealed how much misinformation and outright disinformation U.K. voters were subjected to. Robert Mercer, the right-wing billionaire formerly of giant hedge fund Renaissance Technologies, and backer of political data firm Cambridge Analytica, wanted to bring the same level of mass discontent to the U.K. he helped foment in the U.S. The Guardian called it a "hijacking of the British democracy."
4. Ireland/Scotland complications: Back in 2014, Scotland voted against independence from the U.K. by 55 percent to 45 percent. The Brexit vote was a shocker to the Scots, who also voted to remain in the EU. Further complicating matters are reports that Northern Ireland would be granted permission to stay in the single market. Resolving these issues makes the entire enterprise highly problematic for England.
5. Europe makes leaving impossible: I don't see any reason why the EU would do anything other than make this as uncomfortable for Britain as possible. Basic game theory suggests that the EU's interest lie in the exact opposite direction, and to make exiting as difficult as possible in order to discourage others from departing. There is no reason to expect the EU to reverse course.
Michael Bloomberg, owner of Bloomberg LP which publishes Bloomberg View, referred to the choice of accepting a bad deal or admitting Brexit was a mistake in the first place. The only conclusion one can draw from all of this is that Britain's leaders need to make the responsible decision and stay.
