Re: Not the entirity of China...
@John Savard -
"When Britain returned Hong Kong to Chinese control in 1997, statements publicly made at the time made it clear that "One Country, Two Systems" was limited to 50 years," - Yes.
" and, more importantly, was not part of the essence of the agreement."  - I don't recall that.
But this is besides the point. China made the promise of "One Country, Two Systems" to the people of Hong Kong, in the form of The Basic Law, which the National People's Congress adopted in 1990. It is The Basic Law that says we [I am a British citizen living in Hong Kong with permanent residence, so that includes me] can watch films about Winnie the Pooh (Article 27, Freedom of Speech). However, many people here are worried that the protections under The Basic Law are being eroded before the supposed 50 years are up... for instance, the abduction of booksellers (Article 28, no arbitrary or unlawful arrest, detention or imprisonment).
As to Britain's involvement... no-one here is under any illusion that Britain has the military might to fight a war over HK. I don't think anyone wants a return to British rule... even the Hong Kong National Party (soon to be banned), who often wave the colonial flag at protests, advocate independence, not colonial rule. However, the Joint Declaration was an agreement between China and Britain, therefore, Britain has every right to speak out when China breaks the agreement. Sadly, the value of trade with China and the pressures of Brexit make it likely that Britain will turn a blind eye.