“I‘m not a quitter and there is a long-term job to be done,” she said, according to Reuters. “That job is about getting the best Brexit deal, about ensuring that we take back control of our money, our laws, our borders, that we can sign trade deals around the rest of the world.”

China, the world’s second-biggest economy, could become a more important source of customers and investment for Britain as its scheduled March 2019 departure from the European Union looms. After her meetings in Beijing, including with Mr. Xi, Mrs. May will visit Shanghai to promote trade and investment.

In 2016, China was the destination for 3.1 percent of British exports of goods and services, while 43 percent went to other European Union member countries. Chinese officials are eager to talk up prospects for more business, and British exports to China have been growing. Representatives of more than 40 companies, universities and trade groups are accompanying Mrs. May on her trip.

In a press appearance with Mr. Li, Mrs. May said that she had won China’s agreement to take steps toward lifting a ban on British beef imports, which dated back to the crisis over mad cow disease during the 1990s.

But China also bristles with policies and practices that protect many domestic industries from foreign competitors, and Mrs. May has sounded a more guarded note about prospects for cooperation than did Mr. Cameron and his Labour Party predecessors, Gordon Brown and Tony Blair.

Mr. Cameron and George Osborne, the former chancellor of the Exchequer, courted Chinese investment aggressively, placing less emphasis on contentious issues like democracy, human rights and the future of Hong Kong, a former British colony. In 2015, Britain also bucked the United States by joining China’s fledgling Asian Infrastructure Investment Bank.