Wikipedia says that almost 1 million residents of Hong Kong already emigrated between 1984 and 1997, but not just to the UK. They also go to other Asian nations, the US, Canada, Australia, etc.
I was last in Hong Kong in 1965 and there was talk even then of what was going to happen. By my memory England made deal with CC that did not allow subjects to leave, but as usual it appears fact is stranger than truth.