It was a fantastic coup for Gordon Brown when Joanne, the formidable blonde admired by millions, announced that she was donating £1m to Labour funds. It was on the eve of last year’s party conference, and Brown stage-managed the story, saying that he was “delighted to have her backing”. Meanwhile, a statement was issued by Labour in her name, praising Brown and condemning David Cameron.

This was not, of course, Joanna Lumley: no, the formidable bottle blonde in this case was JK Rowling, the much-loved author of the Harry Potter books. No wonder her support was so highly prized by the prime minister – and not just because it was accompanied by a substantial donation: new Labour has always seen the endorsement of those inelegantly termed “celebrities” as a prize above all others.

Just as companies are prepared to pay almost anything to get such people to champion their brand – think of George Clooney and Nespresso – so the modern political party craves a similar association with stardom, in the hope that a little of the stardust will be sprinkled over its electoral prospects.

It is for this reason that we should have no sympathy with the government’s fury that Lumley, a person elected by nobody and accountable to nobody, has used her star appeal as a deadly weapon against it on the issue of retired Gurkhas’ residency rights.

I happen to share the view of many in the British military: that the bemedalled old Gurkhas knew the deal when they joined up, and that they have been the ornamental front men for a cunningly conceived campaign on their behalf by an astute team of lawyers. But I can still appreciate the beautiful irony of seeing this celebrity-fixated administration being cut off at the knees by an actress who understands better than any Labour party media man how to put image at the service of a political campaign.

Just over a year ago Brown, possibly in an attempt to distance himself from his unashamedly starstruck predecessor, argued that “we’re moving away from this period when, if you like, celebrity matters – people are moving away from that”. He must have forgotten about that broadcast conference call he did with Angelina Jolie, in which the Hollywood actress obligingly expressed the hope that as prime minister Brown would “have a chance to do more and more good things”; and he clearly hasn’t been aware of his wife’s recent involvement with Paris Hilton, after the two women’s attendance at an African first ladies’ health summit (yes, the African first ladies really did think that Paris Hilton would raise the tone of the proceedings).

Sarah Brown’s Facebook page tells the world: “Loved Paris Hilton . . . She’s a smart, caring, considerate person”, while Hilton issued the following on Twitter: “Just had an amazing conversation with Sarah Brown, Gordon Brown’s wife. She is such a smart, beautiful, inspirational woman.” I’m sure both women have summed the other up objectively – it would be ungentlemanly to suggest otherwise – but what is the point of all this, other than for the world of politics and of pure celebrity to hose treacle over each other?

Now that we have established that Brown is no ingénu in the world of celebrity political endorsements, the question naturally arises: why was he so careless of the risks his government was running by studiously ignoring Lumley in her pleas for the Gurkhas? As Lumley devastatingly revealed last week, she had written three times to Brown about the issue without receiving a single word of acknowledgment, let alone a personal reply.

Perhaps we could make the charitable assumption that the prime minister was unaware that No 10 had received these letters from Lumley, and that even now some correspondence secretary in his private office is being defenestrated.

On the other hand, does it not seem equally likely that Lumley comes across as exactly the sort of person that Brown would cross continents to avoid? She is a daughter of the Raj and retains an accent and demeanour that exude effortless social superiority. I’m sure every breathy word she utters brings Brown out in hives – and the sight of her side by side with Cameron will have done nothing to diminish his suspicion: the bloody woman’s a Tory!

As a matter of fact, Lumley’s political affiliations are unknown – she has always been careful to disavow any leanings to a particular party, something that now stands her in good stead as a campaigner attempting to win the broadest level of support. In one sense, however, she is indeed a harbinger of good news for the Conservatives.

When Cameron took over, a number of Labour-supporting commentators expressed the view that such an uncamouflaged member of the upper classes would find it hard to attract the support of the “ordinary working people of this country” (as the prime minister frequently describes the entire electorate). Indeed, Brown initially seemed to think that it was enough to describe Cameron as an “Old Etonian” to bring public contempt to bear on the official opposition. Yet the great success that Lumley has had in inspiring support from readers of such demotic newspapers as The Sun is a clear indicator that Brown is mistaken if he thinks that class is an issue he can turn to his advantage.

It is, though, much easier to be Joanna Lumley than it is to be David Cameron. He suffers from the fact that he is a politician in a period of great public disenchantment with the whole lot of them. Anything he says will be viewed by all but the more faithful Tory supporters with a high measure of cynicism; his main advantage over Gordon Brown lies in the fact that he is not Gordon Brown.

Simply by virtue of not being a politician, Lumley has an overwhelming advantage over her opponents. Thus, she can get away with things for which any politician (except possibly Boris Johnson) would be slaughtered. Last week, when asked how the government should find the £1.4 billion it alleges would be the cost if all the old ex-Gurkhas and their families were to settle here, Lumley, with an insouciant grin, said: “Borrow it!” Everyone around her laughed obsequiously. No wonder Lumley is adamant that she would not want to become a legislator – although I rather like the idea of her taking a seat in the Lords as Baroness Lumley of Kathmandu.

If you doubt Lumley’s wisdom in retaining her political virginity, consider the alternative career of one of our greatest actresses, the twice Oscar-winning Glenda Jackson, who retired from stage and screen to devote herself full-time to public service. For the past 17 years Jackson has been a Labour MP, her high point a short-lived appointment as a junior transport minister.

In 2006 the Glenda Jackson Theatre in her home town was demolished to make way for a new housing estate: such is the affection now felt for the woman who gave up the glories of Hollywood – and regular appearances on the Morecambe and Wise Christmas show – for the less obvious delights of looking after the constituents of Hampstead and Highgate.

It is perhaps not the best time to make this point, in a week when the full extent of Westminster’s corrupt expenses system has been made known, but it would be a good thing if we could learn not to regard all politicians as venal megalomaniacs – or the statements of actresses with wide-eyed credulity.

In the end, political arguments should be judged entirely on the merits of the case, rather than by the relative beauty, charm or fame of the proponents. It will never happen, of course.

dominic.lawson@sunday-times.co.uk

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