This has been said repeatedly elsewhere, but as everyone with any international law background knows, there is no such thing as diplomatic asylum anywhere, except as a local custom in South America. And not all of South America either.
There is no -law- in international law. It is entirely based on consent by each State. Even signed and ratified treaties are still open to interpretation sometimes. Sovereign governments do pretty much whatever they can get away with politically. The highest 'authority' of international law would be the ICJ, because everyone likes to pretend the UN Charter is important. Well, almost everyone. The US doesn't feel like recognising the authority of the ICJ is beneficial, so it doesn't, and then its not bound by it. Simple as that.
It's the same with the UK and Ecuador. The UK does not recognise asylum rights to free passage wherever. Assange is not a political refugee anywhere relevant to European courts. He has a pending arrest warrant. Whatever Ecuador says about their understanding of international law is completely irrelevant. Very simple too.