It's a breach of all of the above. The problem with the Hong Kong joint declaration is that there is no provision for resolving disputes. Since China has not acceded to the world court, the only form of response can be diplomatic, which is one way of resolving international disputes, so that's the case.
I would say not just the U.K., but also Canada, because all of these countries were asked to rely on China's commitments under the joint declaration on the Basic Law as a condition of Hong Kong's special treatment, so once China stops adhering to that, they have every reason to complain. This is not purely an internal affair. There's a kind of partnership over how Hong Kong would be treated, and the partnership is being violated.