I'm glad to report that Jury Nullification is alive and well in Britain; in fact, the judge doesn't even have the power to overrule it. The right to trial by jury is often said to be one of the cornerstones of our democracy; that 'twelve good men and true', twelve randomly selected people, can overrule even statute in cases they decide are being handled wrongly.
This power dates back to 'Bushell's Case' (1670), where it was first stated that the jury were the sole judges of fact and entitled to give a verdict according to their conscience, and can not be penalised for taking a view of the facts opposed to the judge. In practice, this means that juries can acquit any defendant, even when the law demands a guilty verdict.
Some recent cases that demonstrate this power include R v Kronlid and Others (1996), where three women broke into a BA factory and caused over £1.5 million worth of damage to a Hawk fighter plane, because it was to be sold to Indonesia and most likely used to opress the East Timorese; R v Ponting (1985), where a civil servant was prosecuted for breaking the Official Secrets Act when he revealed that a government agency had lied to Parliament; and less politically R v Owen (1992), where the defendant, after exhausting all other, more legal options to attempt to control him, had shot (but not killed) a man who had killed his son and recieved only a year in prison for it, and was continuing to illegally drive a lorry in the same manner as had led to the death of the defendant's son.