The UK felt obligated to retake the Falkland islands after Argentina invaded it, Hong Kongers would have requested the same. At this point would the UK have told China to go and stick it to protect democracy, civil rights, human rights etc?
Chinese tanks would reach the far side of Hong Kong before the RN had passed Land's End and that's if we somehow ended up with a government stupid enough to get us into a shooting war with a nuclear power over a territory we can't maintain without support from the same power...