One of the main attractions at the local museum in Newry, a bustling market town some 40 miles south of Belfast, is an old wooden sign.

Britain’s impending departure from the European Union on Jan. 31 is merely, as Winston Churchill might have said, the end of the beginning.

Since the 2016 Brexit referendum, the United Kingdom has been trying hard to figure out its new position on the global stage. Not without a certain degree of nostalgia, the ruling Conservative Party has put forward the idea of “global Britain,” reviving old imperial networks and repositioning London at their …