I'm not sure it's best to think of that situation as an occasion when the UK made a choice. If the US had said "we wish Hong Kong to declare independence and w

I'm not sure it's best to think of that situation as an occasion when the UK made a choice. If the US had said "we wish Hong Kong to declare independence and will provide military backing as required", I expect the UK would have gone along with it (I offer no opinion on whether that would have worked). But in the absence of that that I don't think there was much of a decision to make. Quite possibly the UK were out-diplomatted and a somewhat better deal would have been available in principle. But ultimately (as we have seen in recent years) there's nothing that stops a superpower saying "we're now going to walk away from this treaty we previously signed".