But in the 1960s/70s when this was all going on, Britain still had Hong Kong and no one was thinking about handing it back to China.
Incidentally, when it came to the Hong Kong colony / dependent territory, Hong Kong island had been ceded to Britain in perpetuity in 1843 as a result of the Treaty of Nanking to settle the First Opium War, followed by the ceding in perpetuity of the Kowloon peninsula in 1860 following the Second Opium War. It was only what came to be referred to as the New Territories that was subject to a 99 year lease from 1898. The problem for Britain come the 1980s was that virtually all the expansion in Hong Kong, and the bulk of the population, were on the leased part. And the expansion from 1965-85 had been huge. Getting rid of the whole thing lock, stock and barrel then became a practical issue.
Britain maintained a naval base in Hong Kong right to the end (HMS Tamar). It has been swallowed up by waterfront developments.