You have to consider which ones are being adopted. It is overwhelmingly the educated and wealthy ones who will go, and a far higher percentage of them speak English.
==About 350,000 UK passport holders, and 2.6 million others eligible, will be able to come to the UK for five years.==
==Under the government's plans, all British Overseas Nationals and their dependants will be given right to remain in the UK, including the right to work and study, for five years. At this point, they will be able to apply for settled status, and after a further year, seek citizenship.==
Do you know (or care to speculate) on how those eligible for this offer would compare to residents captured by the census?
e.g. Did lots of people arrive from the mainland post-97, who might drive down the average? Or perhaps those with overseas-british papers are older, and the younger generation speaks much more English? I don't know enough to guess.
No they don't. According to the Hong Kong government's 2011 census only 41% of residents spoke English moderately well (not even fluently, just moderately well). 20% reported not being able to speak any English whatsoever; not even a few sentences.
Only 57% of residents of Hong Kong considered themselves "bilingual".
Only 30% of students attended an EMI school (English as a Medium of Instruction).
And I doubt the numbers have become more favorable to English in the decade since that census.