Britain laid out a tough opening stance for future talks with the European Union on Sunday, saying it would set its own agenda rather than meeting the bloc’s rules to ensure frictionless trade. After officially leaving the EU on Friday, Britain now must negotiate future trade relations with the bloc, to take effect when a standstill transition period expires at the end of the year. Prime Minister Boris Johnson’s government has been quick to send Brussels a message before trade talks begin in March: Brexit, for him, means sovereignty trumps the economy.