At that time, it was a "lease" rather than a "cut" because in 1898, Russia rented Luda, Germany rented Jiaozhou and France rented Tokyo Bay (now Beibu Gulf). If Britain forcibly cuts back, it will inevitably lead to a rebound in other big countries. Therefore, at that time, the balance of power in China also preserved the formal unity of China in a sense, and it would not be a realistic attitude not to admit this.
During the Sino-British negotiations, the whole territory was in chaos. Under the pressure of public opinion, the British Hong Kong authorities demanded to participate in the negotiations and hold a tripartite meeting, which was then called a "three-legged stool". But it was categorically rejected by the Chinese side. Later, Hong Kong called for a "referendum" to choose between reunification, independence and maintenance. London is optimistic about this, but Beijing will not hesitate to take back the threat in advance by force, so it is time to talk.
Therefore, the two sides can only talk about when and how to withdraw the negotiations, and there is no other option. Hong Kong is unpredictable and can only wait for the final decisions of Beijing and London.
So fortunately in history, it was "leasing" rather than cutting, leaving a "blank" in international law. Then it is entirely up to strength to force the "referendum" to die.
Spain demanded that the Gibraltar issue be solved according to the Hong Kong model, but Britain ignored Spain. Apart from the lack of gap and room for negotiation, Spain itself has no capital to force Britain to sit at the negotiating table. So Britain has nothing to be afraid of, so it plays Spain with a referendum.