Specifically, it takes time to wind it down in a way that does not damage both parties. As you said, Brexit could have happened 100% in a day, but then trade an

Specifically, it takes time to wind it down in a way that does not damage both parties. As you said, Brexit could have happened 100% in a day, but then trade and movement would likely be negatively effected until new deals were worked out. Much better for everyone involved to work out those new deals first, then execute the exit.