That's mostly accurate, but not completely. British criminal legislation often includes explicit defence arguments - 'It is a defence that...' followed by a specific action, belief, or circumstance.
The phone hacking law (RIPA) doesn't include that option.
But the ultimate authority in British Crown Court trials is the jury, and if a lawyer can persuade a jury that a public interest defence is valid, that will swing a trial.
Most of the phone hacking cases couldn't argue public interest because there wasn't any - it was random information gathering that could be useful in news stories and (allegedly) in other ways.
There was no equivalent of the Profumo Scandal of the 60s, which would surely have counted as a public interest case.
(Which is not to say equivalent scandals weren't found, but that if they were the press never revealed them.)
The phone hacking law (RIPA) doesn't include that option.
But the ultimate authority in British Crown Court trials is the jury, and if a lawyer can persuade a jury that a public interest defence is valid, that will swing a trial.
Most of the phone hacking cases couldn't argue public interest because there wasn't any - it was random information gathering that could be useful in news stories and (allegedly) in other ways.
There was no equivalent of the Profumo Scandal of the 60s, which would surely have counted as a public interest case.
(Which is not to say equivalent scandals weren't found, but that if they were the press never revealed them.)