LONDON — Up until now, Britain’s mantra when dealing with the United States’ unpredictable, tariff-obsessed president has been to keep calm and carry on negotiating. That could be about to change. Last week, as London and Washington edged towards a diplomatic rift over Britain’s stance on the war in Iran, the U.K. government quietly set out plans to develop new powers to “protect the U.K. from adverse economic pressure,” warning that trade was increasingly being used as a geopolitical weapon.