LONDON—British Prime Minister Boris Johnson said he would rather find a negotiated settlement to post-Brexit trade problems in Northern Ireland but if the government does escalate its dispute with Brussels it will do so in an appropriate way. Relations between Brussels and London have soured in recent weeks after Britain, unhappy with the deal it signed up to in 2020, threatened to trigger an emergency clause known as Article 16, potentially leading to a trade war. However London and the European Union agreed last week to intensify efforts to solve the trade issues stemming from the Northern Ireland Protocol, after Brussels said it welcomed Britain’s “change in tone”. In a speech late on Monday that was largely about the United Nations COP26 climate summit, Johnson said that he would “rather find a negotiated solution to the problems created by the Northern Ireland Protocol, and that still seems possible”. “But if …