As she embarked on a three-day visit to the Asian giant, May batted away critics of her leadership on Brexit and domestic issues, including within her Conservative party.
"First and foremost, I'm serving my country and my party. I'm not a quitter and there's a long-term job to be done," she told reporters travelling with her.
The House of Lords is scrutinising a key piece of legislation on quitting the EU as a leaked government report shows only economic downsides to leaving the bloc.
"We are determined to deepen our trading relationship even further and we are ambitious for what our future trade relationship will be," May said.
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The British leader said more than nine billion pounds ($12.7 billion) in business deals will be signed during her visit.
The two countries also launched a joint trade and investment review to "identify priorities" on goods, services and investment, she said.
The two sides also agreed new measures to improve access to the Chinese market and remove trade barriers, including making progress on allowing British beef exports.
But she threw in a caveat, saying the two countries would continue to work together on how to ensure that the endeavour "meets international standards".
Touting that Sino-British relations "don't have winter, only spring", Li said their ties would not be affected by Brexit.
"Brexit is a situation that both our countries face," he said. "Our bilateral relationship will not change with the changes of UK relations. We will have assessment and discussion on our trade relationship to make our economic and trade relationship move forward."
Before her trip, the former British governor of Hong Kong, Chris Patten, urged May to address concerns about the political situation in the semi-autonomous city, which London handed back to Beijing in 1997.
In a letter to May's Downing Street office, Patten said Hong Kong was facing "increasing threats to the basic freedoms, human rights and autonomy" which its people were promised after the handover.