Of course Britain is totally unprepared for such a fight
and would most likely lose. They'd need the USA, Australia,
etc to help liberate HK. And that's not going to happen.
This is also a good option.
After making the 3M Hong Kong people citizens of the UK, then UK can claim that the CCP has violated the treaty and re-establish ownership of HK.
Yes, a fight will be required, but maybe its time.
Of course, if we once again recognize the government of Taiwan as the official government of China, give them Chinas seat on the Security Council, things become a little easier......