"a violation of certain norms that the British government is known to follow consistently in practice"
What does "what is known" have to do with what is in fact? The suppressed premise is that citizens know what their governments do, even those parts of the government termed its "secret service." That governments don't operate by ordinary standards of "decency" has been known at least since Machiavelli.
What does "what is known" have to do with what is in fact?
By common usage (and probably definition) "is known" means that the subject is factual and people are aware of it. If people want to make a claim where the subject may not be factual but is still believed then they say 'is widely believed'.
Here's the new thread for posting quotes, with the usual rules: