1. The minimum wage in Hong Kong is only £3.87 per hour. Compare that with £8.72 per hour in the UK, which is more than double.3 million people are unlikely to travel to the UK. First, many love their city even if they hate the political system. Second, Taiwan is closer and probably easier to integrate into - I expect the Taiwanese government would be fairly friendly towards them. Third, assuming Trump loses the general election, the US would also probably be a potential destination for refugees. Fourth, for millions, or even hundreds of thousands, to move at the same time we'd need to see a 1989-Beijing style massacre in Hong Kong.
That said, the UK has discussed with other members of the Commonwealth about the possibility of a joint plan to spread refugees around if the numbers were truly exceptional.
And housing costs are generally a lot lower in the UK than in Hong Kong.
You're looking at roughly 1.8 million people in the 18-35 age bracket - many of whom would leave just as economic migrants.
2. Taiwan won't be welcoming masses of new immigrants.
3. The chances of being killed in the USA (just for being East Asian) is much higher than in Hong Kong.
So how attractive is the USA as a destination?
4. You doubt there would be hundreds of thousands of migrants. But I've already pointed out 1.8 million in the optimum age bracket who could move as economic migrants. So let's say there are 500,000 who would move.
If 500,000 people did move from Hong Kong, I think it would be the pragmatic/elegant solution.
a) Those in Hong Kong who can't accept the situation leave
b) The protests would stop, and business can actually operate with security. Business is the only reason Hong Kong exists and can support that population density.
c) Hong Kong is already grossly overcrowded. Those who remain in Hong Kong would benefit from more living space.
d) At the same time, there are many Chinese people who would be ready and willing to move to Hong Kong, to fill in any gaps.