In theory, the weaker pound should bolster British exports by making them relatively cheaper than those produced by competitors in Europe, North America and Asia.

But given that Britain imports more than it exports, the net effect is negative. Whatever advantages exporters might gain would almost surely be canceled out by barriers to trade across the English Channel if Britain really leaves Europe without a deal.

Most broadly, the decline in the pound signals that investors see less need for British currency in the future, because Brexit is already reducing the appeal of doing business in Britain.

Economists have produced a dizzying array of estimates on the ultimate costs of Brexit, and especially the disruption to trade if confusing new customs checks are established on both sides of the English Channel. A no-deal Brexit would leave the British economy 2 percent smaller than otherwise by the end of 2021, according to a recent report from Oxford Economics, a research institution in London. The hit would be twice as bad by the calculations of the Office for Budget Responsibility, the official British forecaster.

From the auto industry to aerospace, major international companies have over decades set up plants in Britain, exploiting its proximity to the single European marketplace. The more likely a rupture across the English Channel, the less valuable Britain becomes as a base of operations.

None of this is new. Mr. Johnson has merely intensified pressures that have been at play since June 2016, when Britain shocked the globe with its referendum vote in favor of abandoning Europe. The pound plunged more than 10 percent against the dollar the next day. Ever since, the currency’s value has served as a gauge of Britain’s overall economic prospects amid the bewildering wrangling over Brexit.

Inflation resulting from a weaker pound prompted households to limit spending, yielding slower economic growth. Businesses have held back on expansions. Major international companies — Nissan and Honda among them — have shifted production beyond Britain.