A Layman's Guide To Jury Nullification

'The last stand against tyranny can take place in the jury box. The legislative branch may pass unconstitutional laws and the judiciary branch may uphold them. However, the individual citizen has the last say in the jury box. Learn your rights as a juror. Most likely, you will not be told about them in the courtroom.'

Basically, if you find yourself as a juror and you do not agree with the Law that the defendant is charged with/under, you have the right to acquit the defendant accordingly, no matter how the judge directs you. Goes for the UK as well as the US.