> There balance of military power hasn't tipped at all, the western countries just stopped thinking it was okay to establish permanent settlements in other people's countries and then use that to claim we owned it.
Isn't that power? Power doesn't necessarily mean carrying out actual warfare, it means being able to do whatever you want without anyone stopping you. "We could stop them if we wanted to we just ... don't want to, yeah that's right..." rings pretty hollow. The fact that zero enforcement action has been taken means China is more powerful in the region.
All the UK can do to enforce the handover agreement is write strongly worded memos that get sent right into the shredder in Beijing. Similarly with the islands in the South China Sea. The response is basically "what are you going to do about it?" Not what could you do about it.