The UK should have offered citizenship to residents of Hong Kong prior to 1997. It exploited the colony and local residents of HK as much as China does now.

Offering them citizenship will allow residents a way of moving to the UK, but it does not guarantee them consular rights in Hong Kong. China already takes the view that a HK Chinese resident is a citizen of China, subject to its laws irrespective of other citizenships and will not allow consular assistance.

As for Taiwan, Nixon and other swapped "allegiance" from the ROC to PRC in 1972. Most nations do not havce diplomatic relations with the ROC because if they do, then PRC withdraws diplomatic recognition and negotiation with them.

Note that Taiwan's government does not want "independence". They still claim to be the legitimate government of the whole of China, even though that status was lost in the UN in 1971. Both the ROC and PRC claim that Taiwan is part of China, but differ (obviously) as to which government is "legitimate".