Hi James - when did the UK prosecute someone for calling Scientology a cult? Do you mean the scientologists took them to court?
There have been several threads about it here. As I understood it, it was not the scientologists that charged them, but the British government itself. They were marching with signs saying "Scientology is a cult" outside a Scientology property - & were charged with hate speech.
It was the discussion on these threads that caused me to realize that the British do not have a specific "freedom of speech" constitutional right like the United States does.
It is true, though, that the British do not have as much of a "hands off" policy toward religious finances as the U.S.