LONDON — Britain’s Treasury chief sought Monday to ease concerns about the vote to leave the European Union, saying the economy is as strong as it could be to face the uncertainty — even as a survey showed many companies are looking to move business out of the country. In his first public appearance since Thursday’s referendum, George Osborne stressed that Britain’s economy is in a far better position than it was at the start of the 2008 financial crisis. “It will not be plain sailing in the days ahead,” he said.