Park a carrier strike group offshore, guarantee safe passage and refugee status to any Hong Kong resident wanting it, and provide transport to a host country.
It would take tremendous political will and resources. But this would be the non-violent, humanitarian and decisive way to intervene.
At which point do you think the first bullet of the war fires ?
Sure, it's not like anyone can or willing to do much about that, but the UK can play the Russian game (with some help from NATO), and provide help for the BN(O) people.
Shooting would be almost inevitable, but that at least establishes that there is a problem.
And why would Xi risk WW3, he and China has the most to lose, after all the future, with the current trajectory of the world looks to be theirs!