If we were to accept May's deal as it was in December, the UK would leave the EU but remain entirely within the custom's union. Afterwards, plans would be made for NI to stay within the custom's union and the rest of the UK to leave. However, there is a term in the 1800 amendment of the the 1707 Acts of Union that says:
“that in all treaties made by his Majesty, his heirs, and successors, with any foreign power, his Majesty’s subjects of Ireland shall have the same privileges, and be on the same footing as his Majesty’s subjects of Great Britain.”
If this term is broken, it reverts all the countries of the UK back to individual states - i.e. Scotland would be independent by default and there is a good chance they would not vote for an amendment to the Acts of Union because they would prefer to stay in the EU.
That leaves politicians with two options: Either keep the UK in the custom's union indefinitely, or break up the United Kingdom.
https://www.thenational.scot/news/16989865.this-is-why-westm...
The Scottish seem to think that when seceding from a Union is a legitimate option, and if a referendum can trigger a secession from the European Union, then, well, what about those 1707 Acts of Union with England? I believe this will happen come April, and that the current pro-brexit crowd will promptly label it undemocratic and unpatriotic - after all, breaking free from the yoke of Brussels is Something Completely Different than rejecting the friendly rule of Westminster. Right?
Madness, I tell you, with only more madness in sight.