The British were not promised that. They were promised after decolonization, Hong Kong would get elections for 50 years and then China would fully take over.

The British were not promised that. They were promised after decolonization, Hong Kong would get elections for 50 years and then China would fully take over. China was also allowed to implement laws protecting national security. What the British didn't foresee is China being able to gerrymander and also influence which politicians could run in the name of national security.