You are correct, and the current (mostly non-)responses from the British government are embarrassing.
However this remains a public treaty, and if China does not respect it, it will erode their standing on other matters (e.g. why would Malaysia/Vietnam/Philippines sign any treaty regarding the China sea dispute, if Beijing ignored the one on HK?). So yes, HK is in a bad situation, but not all is lost yet.
Your reply ignores the realities of power in the South China Sea, just as it ignores the realities of power in Hong Kong (what sort of response would you like to see from the British government?)
However this remains a public treaty, and if China does not respect it, it will erode their standing on other matters (e.g. why would Malaysia/Vietnam/Philippines sign any treaty regarding the China sea dispute, if Beijing ignored the one on HK?). So yes, HK is in a bad situation, but not all is lost yet.