All of the HK basic law with advantages and disadvantages were written by China, who also has the right to amend it at will. This is written in constitutional law regarding China's SAR territories.
Decades ago, China had made a joint communique with the UK, in order to normalize relations. Then, China said it wished for HK to have a high degree of autonomy for 50 years, which again, the definition of autonomy is completely decided by Chinese law.
This joint communique is no less ironclad than say the US-China ones about Taiwan Island. China does not owe UK any particular services because UK invaded more than a century ago and were made to surrender their illegal gains decades ago. Just like US is not legally entitled to help Beijing actively fight Taipei, the communique between Beijing and London doesn't legally force Beijing to help London enforce its own dystopian and regressive ideals on a vibrant Chinese city.
In fact, British interference in HK breaks the guidelines set down by Deng and Thatcher long ago. HK's autonomy is supposed to be both from communist and British absolutist monarchy influences. By sending in and funding armed "volunteers" including the fascist Ukrainian Azov, UK and US broke the 50 year peace and independence of Hong Kong and voided the joint communique themselves.
