afaik, no country can unilaterally legislate to break the terms of an international treaty
Otherwise, no treaty would ever be worth negotiating
This looks the daftest Brexiter idea yet and - if it actually happens - would be the likely start of a trade war,
while the EU goes to the World Court at the Hague to sue the UK
The current negotiations breaking down would be a storm in a teacup compared to breaking a treaty
What country would bother negotiating with a UK govt that breaks a treaty because it didn't bother to read it before signing