Dispossessed Exhibition Launch at The Museum of London
Chris Huhne with Carina Trimingham Photograph: Alan Davidson/The Picture Library Ltd/Alan Davidson

As soon as a British politician imitates a Bible-belt Republican and puts pictures of his adoring wife and chubby-cheeked children on his election literature, you can guess what will dominate the next "news cycle". The press will reveal that while the leaflets were at the printers he took a campaign worker behind the filing cabinet, buried his head in her hair and whispered words to the effect of: "My wife doesn't understand me." If he lacked the smallest trace of imagination, he will actually have said: "My wife does not understand me." Meanwhile, unless the wife possessed a capacity to forgive rarely found in the human species she would not take news of the affair well.

The literature Chris Huhne distributed to the voters in the 2010 general election showed a picture of Huhne and his wife, Vicky Pryce, as newlyweds. "Getting married does not seem like 26 years ago," he wrote underneath. Another faded snap saw the young couple holding a baby. "Families matter so much to me," said Huhne. "Where would we be without them?"

Chris Huhne was not so much tempting fate as lying flat on the floor and inviting it to trample all over him.

Sure enough, the tabloids reported that while he was telling his constituents of his devotion to hearth and home, he was also tending to the needs of one Carina Trimingham, his campaign press officer. Huhne responded to the exposure by dumping his wife of 27 years. "The quality of mercy," did not as Shakespeare had it, "droppeth as the gentle rain from heaven" in this instance. Or if it did, it missed his wife. The rage of Ms Pryce was Vesuvian and the vengeance she wreaked on the energy secretary's career a wonder to behold.

The outcry that followed has shown much that is dismal about British public life and a little that is good. The small comfort is that the police investigated the accusation that Huhne falsely informed the authorities that his wife had been driving a car that had allegedly been speeding so that she could take the penalty points and he could keep his licence.

The real scandal behind the phone-hacking inquiry is that the elite thought that it had to keep Rupert Murdoch sweet and not investigate criminal charges against his employees too vigorously. Although Lord Justice Leveson is yet to investigate the corruption of government, at least police and prosecutors have learned their lesson.

If Huhne had been a private citizen, two forces and the DPP would not have spent eight months on an allegation, denied by Huhne, that caused no death, injury or damage to property. They may have broken a butterfly upon a wheel. They may have been goaded by a rightwing press, which hates Huhne because he has the integrity to say that global warming is a real phenomenon that we must tackle. But most people in the world would love to live in a country where the law treated accusations against the powerful seriously – or indeed at all.

After that, however, the reasons to be cheerful vanish. A small point that may only bother journalists is that the police obtained a court order against the Sunday Times in the autumn demanding that its political editor hand over confidential emails from Vicky Pryce.

The Sunday Times said it would defend its source and appeal. According to the DPP's statement on Friday, the case against Huhne and Pryce moved towards prosecution because the Sunday Times "consented to producing the material in question just before the appeal was due to be heard, on 20 January".

The maxim that journalists must never reveal their sources is about the only moral principle we have. At its noblest, it recognises debts of honour. Informants give you information in the public interest and say that their career, liberty or life depend on keeping their name confidential. You promise to protect them and must keep your word. More prosaically, journalists reason that if we reveal sources, other sources will not take the risk of speaking to us in the future. The Sunday Times said it told its source it would protect the source's identity unless a court ordered it to hand over any material. Perhaps its lawyers said that the struggle was hopeless, but the fact remains that once journalists went to jail rather than make their sources public. Now they won't even go to the Court of Appeal.

Meanwhile, in another court, the bisexual Carina Trimingham began privacy proceedings against the Daily Mail. She claimed that it had published "inherently private" information and described her as a "comedy lesbian from central casting". We await the judge's ruling. But the willingness of the judges to consider her case, even after they provoked the scorn of public and Parliament by allowing Fred Goodwin to claim that the affair he was having at work while he was driving his bank over a cliff was a private matter, shows how little the judiciary understands the needs of a democracy.

Free societies are raucous places. They do not always conduct themselves in the best possible taste. In the US, with its constitutional protections for freedom of speech and the press, Goodwin and Trimingham would have been unable to bring a privacy action. They were public figures involved in public controversies and would have had to argue in free debates without calling on the judges to help them.

Trimingham was Huhne's campaign press officer and mistress when his campaign literature presented him as a wholesome family man. After he moved in with her, she touted for work with lobbying firms, telling them that she could get their clients in front of senior members of the coalition.

In these circumstances, she should have had to defend her reputation before the court of public opinion, not attempt to suppress debate before a court of law.

There is an argument that we do not need American freedom because there is no public interest in sniffing Hugh Grant or Sienna Miller's dirty linen. I would accept it if we had judges who overrode privacy rights and allowed publication when there was a public interest in exposure. Unfortunately, such judges are hard to find in Britain and we have a legal system whose priorities could not be more awry if it was standing on its head.

One branch of the law makes public what should be kept secret. Another keeps secret what should be made public.