The most practical outcome seems to be a gradual emigration of all the HK citizens to the UK/other countries. I can imagine citizens forming networks where they would warn each other of an upcoming arrest so their friends/family would scurry out of the country.

I am hopeful that the sheer number of citizens in HK would perhaps lead to the city retaining some of its independence. We can't predict the future... who knows? What if Xi dies in the next 5 years and his successor isn't such a hardliner?