For the simple reason they are the other signatory to the international agreement China is breaching. China's word is worthless if they won't stick by a legally

For the simple reason they are the other signatory to the international agreement China is breaching. China's word is worthless if they won't stick by a legally binding arrangement they freely entered, and entered as the party with most leverage. What does Brexit have to do with that? Nothing. Governments have multiple department and often do multiple things at a time. That's not exactly revolutionary.