About 3.4m Hong Kong people have British National (Overseas) status and can immigrate to the U.K. tomorrow should they wish to do so. I imagine a substantial number of those not entitled to BNO are children of those who are and would be able to migrate with their family if their entire family leaves. All the Chinese who moved to Hong Kong after the handover are screwed however.

https://en.wikipedia.org/wiki/British_National_(Overseas)

> Applicants were required to be British Dependent Territories citizens by a connection with Hong Kong.[30] While about 3.4 million people acquired the status,[41] 2.5 million non-BDTC residents (virtually all Chinese nationals) were ineligible.[42] Those ineligible who wished to register as BN(O)s were required to have been naturalised as Hong Kong-connected BDTCs by 31 March 1996. Acquiring Hong Kong BDTC status other than by birth was no longer possible after that date.[37]