Former British Prime Minister Tony Blair is pressing his long-held view that voters should be given a chance to re-think Brexit once plans for Britain’s departure from the European Union are clear. Blair said Thursday that the Labour Party he led for 13 years should be actively challenging Brexit rather than going along with plans laid out by Prime Minister Theresa May and her Conservative Party allies. May’s government is negotiating exit plans and the outlines of a new trade relationship with the EU with an eye toward departing the bloc by a March 2019 deadline. Blair and other opponents...