One of the reasons UK didn't contest it in 1997 was that it couldn't. UK "owned" Hong Kong island, which is a tiny bit of territory. Most of what is called Hong Kong was actually leased from China for a definite term, and the lease was coming to an end, fair and square. Hong Kong island was handed over as part of the package.

Hong Kong island is, I would imagine, in no way sustainable as a standalone territory, if China were to be hostile.