One of the major phenomena of Hong Kong in the 1980s and early 1990s was families getting out if they could afford it, because everyone was nervous about the Chinese handover. This wave of emigration even plays a role in contemporary Hong Kong film of the era like Chungking Express. And even before the fears of the handover, many educated HKers were emigrating to the US for the sake of higher wages.

Yes, HKers have ties to HK that are a hassle to break, but plenty of people have broken them and up and left.

Most HKers are descended from mainland Chinese that crossed the border after WW2 in the 50s and 60s. Just mirroring their ancestor's footsteps.

The sentiment in the mainland is more or less get out and leave the us the land

To be honest I think that the Chinese government would not care or actually be quite OK with it.

Indeed, who would leave HK? Arguably the anti-CCP, the "troublemakers". So that'd mean less 'trouble' in HK and there is a large supply of mainlanders happy to move to HK.

Obviously the Chinese government will officially be outraged but privately they may think good riddance.

Talking to a Chinese friend who is quite pro-CCP, they are pissed that UK is offering this and say that mass emigration of HKers would be a major loss of face for the CCP.

Yes, but that does not mean there would not be a silver lining.

This offer of residency is political and possibly symbolically negative for China, but in practical terms it does not hurt China at all.