Note there's now a great deal of drum-beating going on to close the border to "non-residents" (that's a Hong Kong euphemism for Mainlanders).
https://www.scmp.com/news/hong-kong/...d-taiwan-close
While in fact closing the border to "non-residents" will have little benefit -- with the mandatory quarantine in place there aren't that many non-residents entering Hong Kong now; the real risk is returning residents quarantining with family members who are free to circulate in the community -- I suspect the Government will do something along these lines. So your decision to head to Hong Kong ASAP is a wise one.
This is the most expensive ticket I have ever booked
But I think it was the "right choice" - not that there is a right or wrong choice but UK is really locking down too. And as you said, it is a no brainer to shut the borders to HKG