Brexit Is Finally Done, but It Already Seems Out of Date

LONDON — It took 11 grueling months for negotiators from Britain and the European Union to hammer out the terms of a post-Brexit trade deal. But in many respects, the deal is already four and a half years out of date.

The world has changed radically since June 2016, when a narrow majority of people in Britain voted to leave the European Union, tempted by an argument that the country would prosper by throwing off the bureaucratic shackles of Brussels.

In those days, the vision of an agile, independent Britain...