Georgiana is the perfect British heroine. The Duchess of Devonshire was glamorous, articulate and daring, the doyenne of the Whig party in the 18th century and a devoted mother who gave up her future for her children. She was painted by Gainsborough and Reynolds and adored by everyone from an Irish dustmen to dukes (except her own).

Charming, tragic and ultimately subservient, as Amanda Foreman shows in her bestselling biography, she was how we like our women. In the film released today, Keira Knightley plays her early years to pouting perfection. Rejected by her husband, Georgiana had to end her affair with Lord Grey and died at 48 from an abscess on her liver having given away her illegitimate daughter.

She chose her family over her ambition and happiness, and was never more than an entertaining sideline in British politics, sleeping with the future Prime Minister rather than setting the agenda.

These kind of women are still seen everywhere in British life. Diana, Princess of Wales, Kate Moss, Tara Palmer-Tomkinson, Amy Winehouse, even Knightley off-screen teetering down the red carpet in her heels, tongue between teeth.

They excel at parties, dabble in humanitarian politics and are adored and dismissed by the public for their dazzling, often destructive lives.

A report published yesterday shows how little has changed. Georgiana was well educated and accomplished; girls in 2008 make up 54 per cent of university entrants. Yet now, as then, academic accomplishment is no guarantee of career success.

According to the Equality and Human Rights Commission report (chaired by a man with the help of women), the number of women in powerful jobs is actually diminishing. Fewer than 9 per cent of the senior judiciary and 11 per cent of directors in FTSE 100 companies are women and only 19.3 per cent of MPs are female.

The Duchess was criticised for involving herself too openly in politics. Women in Whitehall today still have to try to influence behind the scenes. Take the Chancellor's interview last month at his Scottish croft. The most telling aside came from his female aide. “Make sure you tell them everything,” she said. His wife also encouraged him to speak his mind, while she got on with the job of vacuuming up the midges.

That is how we prefer our women in the House of Commons. Nancy Astor may have been given her own peg there 90 years ago, but, according to the report, a snail could crawl along the Great Wall of China before women are equally represented in Parliament.

Women, the report argues, are being buried under a “concrete ceiling”. It is not just men who are mixing the cement. When I interviewed the Olympic cyclist Rebecca Romero this week, it was my female colleagues who assumed that she must be a nutter because she appeared so obsessed by winning gold.

It is easy to point to the failure of those women who have been selfish enough to try to fulfil their potential and have a family. Jacqui Smith is derided as Home Secretary, Ruth Kelly was demoted to run the Department for Transport, and Caroline Spelman, the Tory chairman, remains embroiled in the Nannygate affair.

But the truth is that many of them have got into trouble because they were given too little support, too much scrutiny and promoted too quickly as their parties scrabbled around for female faces. If there were more women MPs, there would be more choice, and they could redefine the role. Yet women are not rushing into politics because they see what happens to those who do. Margaret Thatcher is now considered an aberration, a role model for Gordon Brown or David Cameron, not a mentor for women.

That is why the moose-killing Sarah Palin is not a joke. She believes in her family and hobbies, but she wants a mainstream career, not some alternative, part-time, flexible, self-employed affair. Unlike Hillary Clinton, she doesn't feel the need to match the boys except with a rifle: she is just as comfortable waving her breast pump.

The Republicans seem happy to let her do it her way, to be a hockey mom, a wife to First Dude, a governor and maybe even a vice-president. Her aides rather arrogantly pointed out that they would have to rewrite the speech completely for a woman, but in the end those were Palin the pitbull's words yesterday as she said: “This is America and every woman can walk through every door of opportunity.”

In Britain women aren't taking the top jobs because we think that to succeed in corporate and political structures we must make the same choice that Georgiana was offered and refused. Give up our children, surrender our roles as mothers and play by the same rules as men.

Mrs Palin shows that you can want to be a wife and a mother as well as a politician or businesswoman. You may not be perfect in any of these roles, and in her case you may not be right about many issues, but the only concrete ceiling in our public life exists in the minds of British women.

Georgiana, chignon and all, would have been cheering Mrs Palin on.

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