The Uk government established certain rights for countries that were part of the Commonwealth, included residents of Hong Kong in the nineteen sixties. It was basically to remove any right of residency in the UK. It is unlikely that will change. It is believed some people voted to leave the EU to stop immigration. it would not be popular to extend the right of citizenship to several million potential new immigrants.
The Chinese would not be too upset if millions of anti-government residents left, However, it would probably be seen as interfering in Chineses government policy by the Chinese.
The British have a large investment in Hong Kong. I suspect protecting investments will out weight human rights issues.
Also, the UK has been sounding out possible new trade partners, including some old commonwealth countries. India has raised the right of residency issue. It's a can of worms the UK government is keen to keep sealed.