Matthew Single was expelled from the party, where he held the voluntary position of deputy security officer, in 2007. His wife, Sadie Graham Single, worked for the party and was arrested but the charges against her were dropped.
In 2008 Single leaked the party's membership in a blog. Members later complained of harassment, threats and of swastikas being painted on to their houses. The court heard that 160 complaints were received.
One policeman lost his job over the revelation that he was a member. Since 2004 police officers have not been allowed to promote or join the BNP.
Single, who is unemployed, was fined £200 and ordered to pay £100 in costs.
Judge John Stobart expressed surprise that he could only impose a fine on the man. "It came as a surprise to me, as it will to many members of the party, that to do something as foolish and as criminally dangerous as you did will only incur a financial penalty," he said, according to the BBC. "The fine is so low as to be ridiculous."
Also among the people named as members were servicemen, doctors and teachers. The judge said that regardless of their views the people named deserved the protection of their organisation when it came to confidential information.
"While there may be some members in this organisation who do not deserve to be protected by the law, they should be able to expect that officers within the organisation will not abuse the information provided to them," he said. "The law exists to save people from such revenge attacks."
A police spokesman told reporters outside the court that he, too, was disappointed with the sanction.
"There was pretty serious stuff after what happened. People were fearful for their safety," said Detective Sergeant Chris Reynolds. "There was an arson attack on a vehicle, there were daubings and malicious communications. White powder was also put through people's letterboxes purporting to be anthrax and there were daubings of swastikas on garage doors and on homes."