From Wiki:
Before 1983, all citizens of the British Empire held a common nationality. Citizens of the United Kingdom and Colonies (CUKCs) had the unrestricted right to enter and live in any British territory.
CUKC好似只係居留權, 唔係直接BC.

不過都唔太樂觀. 因為CUKC包括其它殖民地. 唔只HK, 會有好多人, 所以英國當年先會修例.
CUKC 去英國都可能要坐移民監.

CUKC->BNO 要還原都唔係咁簡單, 可能要申請. 如果自動恢復, 咁好多高官咪自動有英國居留權

等英國修改法例+中國反對, 睇怕都到左2046, 到時BNO班人都已經送晒去集中營 .