It is not clear what exactly you are asking. One thing is the knowledge AVAILABLE, I mean available in principle, to any Westerner who takes an effort to search it. Another thing is how much a random person in the street, who reads major newspapers knows. I do not think Orwell had special access to any information that was not available to other writers on Soviet Union. The problem is with what they wanted to know and see. In all periods of existence of Soviet Union, there were people who escaped from it and published books in the West. But "general public" either did not believe them or did not care.
The fact is that "public opinion" was deceived by Soviet propaganda,
very much helped by the propaganda of many left-wing Western writers of high standing.
Most people in the West do not realize that Soviet Union (and Modern Russia)
has an enormous State-sponsored and highly effective propaganda machine on the scale unimaginable in the West.
To put is short: adequate reports of what is happening in Soviet Union were always available in the West, since the very beginning of Soviet Union. But they did not receive wide attention, for several reasons which I outlined above. And were unknown to the "broad public".