It's worth noting that the Qing Dynasty ceded Hong Kong Island and Kowloon to the UK in perpetuity. The lease was for the the New Territories, without which it was generally accepted the city would not be viable. Hong Kong was (and still is) dependent on the Chinese mainland for imports of natural resources such as fresh water, so if the Chinese decided to play hardball the British had few options. And that's not even considering the possibility of China deciding to retake Hong Kong by force.

> Even Thatcher (hardly a shrinking violet when it came to using force to get her way) couldn't get a deal to continue British rule in Hong Kong.

Deng actually told Thatcher that if britain didn't leave, china would invade HK and unify by force. Thatcher actually asked the US if they would protect britain from china and we said no. And that was the end of britain's hope of keeping stolen territory.