RousseauX
Donor
As is always the case it depends very much on what replaces the CCP. If it is bad enough (Khmer Rouge bad) the international community might be willing to recognize the RoC as the legitimate government. If the international community continues to recognize Beijing, regardless of the landlord, as the legal successor state then the UK is very much over a barrel.
Giving up Hong Kong to a bunch of heavies isn't going to sell very well to the voters, but without international support, and big bunches of it, most especially including the U.S., Commonwealth, and most of the EU, the British simply don't have the resources to resist the change over without WMD. Can't see that happening.
If it's Khmer Rouge level bad I can't see the mainland government actually being stable enough to go for HK.
If it's something more moderate I think the UK/US might be a lot more lenient in allowing HKers to immigrate before the handover.