I'm pretty sure China considers that as "signing under duress" and therefor invalid.

The relevant points on the timeline, from China's perspective, are:

China: Stop selling opium in our country. UK: How about no? China: We're kicking out your drug dealers. UK: How about an Opium War? China: Oh crap, you have way more guns. We surrender. UK: OK We're taking HK for 100 years. China: I guess we don't have any say in the matter....

A few years later... China: We get HK back now, right? UK: Yeah but we've altered the terms. Take it or leave it. China: OK. I guess.

A few years later... China: Now we have more guns so here are the new terms. Take it or leave. UK: But our deal!!