I disagree with the majority of you.
Something can be not illegal (therefore legal). i.e. self harm.
If the case is appealed and appealed, eventually it will reach the House of Lords. When it does so, the house of Lords, who have the ability to make new law, can then say that the offence (self harm say) is not in the public's best interest, and thus find the accused guilty.
However, inmost instances, I would agree with the above posters.