Why fashion editors love Sam Cam and (very slightly) despair of the Duchess From a sometimes staid princess with no template to follow, to a prime minister's wife who has ditched frumpery for chic, both are, in their own ways, trailblazers. Not since Michael Marks and Thomas Spencer mounted their assault on the nation's housekeeping budget have two brunettes had such an irresistible impact on British fashion as the Duchess of Cambridge and Samantha Cameron. Some of the financial hoo-hah may be dubious (January's announcement that the Duchess of Cambridge was worth a billion pounds to the UK high street was a suspiciously neat figure). Yet even allowing for the endless churn of prose so purple it could shame a Liberty's carrier bag (prose issuing, it should be said, from fashion brands the two have patronised, rather than their own offices), the Kate and Sam show is remarkable. When was the last time we had two posh women exporting a vision of UK plc that didn't involve Downton Abbey or Daphne Guinness? Kate Middleton's fashion hits and misses in pictures Both boast glossy hair, disciplined diets, unofficial stylists, L K Bennett and Russell & Bromley court shoes... to the untrained eye they're cut from the same mould. They often shop at the same places, too. But the devil's in the subtle details. Tribally, they're from different Britains. Sam Cam's neon pops of colour are west London by way of an art course at the University of the West of England; the Duchess is county-meets-Mahiki through and through, an elegant yet modest medley of skinny drainpipes and ruffle-edge cardigan, neat, tailored dresses and suits. Plus jaunty hats of a type not worn by women under 55 since the late Princess of Wales jettisoned them, circa 1989. Occasionally, the Duchess nods to high fashion - a peplum, or a polka dot Topshop pencil skirt. These nods, though, are cautious, always accentuate her slender figure and are always unimpeachably classic and eminently recyclable. British designers seem to be mostly saved for Abroad, possibly because Abroad is less likely to raise merry hell about the price and she can offset the expense against diplomatic brownie points. Wearing her navy lace dress by Canadian-born, London-based Erdem on her trip with Prince William to Canada was a perfect illustration of high-end fashion becoming a useful linguistic alternative to "I'm very glad to be here." The Duchess of Cambridge wore a dress by Canadian born, London based designer Erdem. Apart from the time when she dressed down in jeans to visit a school in Islington last autumn and the children remarked wistfully that she didn't look like a princess, she hasn't put a perfectly shod foot wrong. Unless, that is, you think her role is consistently to showcase the best of British creativity rather than a median average. Plenty do. Plenty more admire the Duchess's sense of the appropriate. Whatever "appropriate" means for a young woman married to the future king of Great Britain. Samantha Cameron and Michelle Obama go head-to-head in fashion showdown The last time we had a woman in Kate's shoes it was the Eighties, a decade famous for appalling style solecisms but also, in comparison with today, far more formal. The first time Diana and Fergie discarded their tan tights to go into the Royal Enclosure at Ascot bare legged, it made the front pages. These days Kate makes the front pages for wearing flesh-coloured tights. And it's hard to imagine Diana ever sending an aide to shop in a second-hand "dress agency" - the Bucklebury equivalent of vintage - on her behalf, as Kate did. But times have changed. If the Duchess sometimes seems oddly out of step with every other 30 year-old in Britain, it's because there's no template, sartorially or otherwise, for her to follow. Zara Tindall? The Duchess isn't really a fleece and jodhpur, tight dress and experimental shoes girl and she's far too fastidious to turn up to a wedding as Zara did in a sundress and visible tan lines. The Gucci'd up-Grimaldis? Major style, sure: but minor substance. Princesses Beatrice and Eugenie? Er, pass. No doubt about it, Kate - and/or the Palace - are having to make it up as she goes along. Millions across the world are enjoying the view, including particularly young women who like this demure alternative to the increasingly sexed-up images of the last 15 years. At night her floaty, embellished, elegant columns in sweet pastels, neither buttoned up nor too revealing and sweetly colourful, are just what you'd want from a princess. Compare and contrast these sugary confections with Sam Cam's fashion-forward moment at the White House last month. In sartorial terms, that high-necked, midnight blue lace column evening dress by the hitherto little-known London-based designer Alessandra Rich was as divisive as the proposed National Health Service reforms. Not only that, she substituted her own belt for the jewelled one that came with it, a knowing gesture that fashion types call Giving it Some Edge. For fashionistas it was a bona fide Moment, both on-trend and dignified, just like the cyclamen pink Roksanda Ilincic outfit she wore to a Number 10 party she hosted for the fashion industry in February. Not everyone appreciated the discreet charms of the Rich dress, however. A quick reccie of the comments posted under a piece I wrote about it at the time suggests that an equal number, if not more, deemed it a frumpy abhorrence. The Duchess of Cambridge and Samantha Cameron have both championed London based designer Roksanda Ilincic. Never mind. In Grazia and Vogue circles, they're still pinching themselves because a British prime minister's wife has turned out to be a poster girl. A mother of three who works in the luxury-fashion business - and knows how to rock voluminous shapes with skinny silhouettes instead of always resorting to the simpler option of dresses and matchy-matchy suits - is an unprecedented blessing. She's a shinier, wealthier version of us: the girl next door, if your spiritual home is W10, that quintessentially metropolitan interface between pretty Notting Hill and gritty Kensal Rise. She won't be faced down by Chanel-wearing French first ladies come photo opportunity time. We've finally got our own Carla Bruni - without the high maintenance and history of weird relationships. If Sam Cam is worldly and metropolitan, the Duchess shares her in-laws' genius for engaging with ordinary people in the real world. She's not affiliated to any brand; she can't accept freebies. That USP alone - untainted by the Mammon touch of advertising - makes her extraordinarily effective. Whether she really shifts a gazillion pairs of nude-coloured shoes, patent espadrilles or sparks a stampede for Whistles' cream blouses, navy cropped Zara jackets, royal blue wrap Issa dresses, cocker spaniels, coral-coloured jeans and J Brand drainpipes is far less important than the image she's selling of UK plc. Fit for purpose, approachable, pretty, reassuring, dependable and decorous - by comparison, Michelle Obama's terrifyingly off-message. First Lady fashion: Samantha Cameron vs Michelle Obama That's why grumbles that the Duchess is too staid or unsophisticated - the maple leaf hat she wore on that trip to Canada is often cited as a particular shudder-inducer in fashionable circles - miss the point. She's influential not because she's a trailblazer but because she's a highly aspirational version of normal. If she can wear it, Reiss's stockroom permitting, so can we. There are grumbles, though. There would be. You can't please everyone. For some, the ecstatic Fashion Moment of that McQueen wedding dress, when cutting edge and tradition magically coalesced, has been followed by a steady slide into cautious, consensus dressing. She is John Lewis and the fashion world would like her to be Harvey Nichols. They'd also like her to use her high visibility to support more young British labels, a number of which still manufacture in the UK, rather than flying the flag for the relentless efficiency of Chinese or Romanian-produced high street clothing. The Duchess of Cambridge in a Maple leaf hat on her trip to Canada. The eagerness for any royal to be seen to be in it with the rest of us is new and arguably there's something calculated and condescending, not so much about the frugality itself, but the persistent emphasis on it, especially given that the Duchess has apparently returned high street purchases once she's been photographed walking down the street with the carrier bags. But most of the other niggles voiced - usually off the record - about everything from the passé length - or lack of - of her skirts to the density of her kohl could be brushed aside as the usual taste issues that split the cognoscenti from the mainstream... except that sometimes the criticism is masking deeper reservations. Jenny Packham: 'The Duchess is a dream to dress' If Diana was the perfect cipher for her times, the Duchess - whom the palace is intent on presenting as Not Diana - is something of an old-fashioned anomaly. Maybe what really bugs observers isn't so much the staid, unadventurous clothes as the idea that she herself might be staid. In the end, the fault lines around which Kate and Sam gather their style artillery aren't the traditional British ones of class, but a much more modern debate about what it means to be your own woman in the 21st century. Samantha Cameron's high-profile career at Smythson, where she was creative director (and still goes in two days a week as a consultant), gives her a latitude that the Duchess of Cambridge probably doesn't have. It's not just that Sam Cam can wear trousers to formal events or go all-out republican - hatless (even to Kate's wedding) - and the Duchess apparently can't. As a director of a high-end label that makes expensive handbags, Samantha Cameron has justification for not compromising on quality in order to make political points about abstemiousness. She got her own Pastygate moment out of the way early on - a spotty M & S dress she wore to conference one year that turned out to have been specially made for her. Her high-low style manifesto, incorporating offbeat high street names such as Cos, a favourite of fashion editors (her sister Emily is deputy editor of Vogue), and quirky pieces from under-the-radar designers such as Emilia Wickstead and Alessandra Rich, resonates with fashion-conscious women who want to look interesting but not ridiculous. Why Samantha Cameron's Alessandra Rich White House gown is a winner with the fashion pack Under her creative oversight at Smythson, which she helped transform from heritage stationery brand to a desirable luxury accessories house, Sam Cam was well placed to accept the British Fashion Council's invitation to become an ambassador for London Fashion Week, a newfangled post that could have opened a prime minister's wife to charges of elitism and frivolity. Significantly, she hasn't only got away with it, she's helping change long-held perceptions of fashion as expendable frippery to bankable national asset. The Downing Street party she hosts each season for the fashion industry has proved irresistible bait to international press and buyers and there isn't a designer who wouldn't like to dress her - this in a world that, for all its entrepreneurial spirit, doesn't normally find itself warming to the wives of Old Etonian PMs. Considering the traditional sartorial destiny of Britain's political wives, the frumpy (and not in a charmingly deliberate Prada-esque way) tailoring, helmet hair, school-of-hard-hosiery-choices and the inevitable showdowns with showier, more polished foreign first ladies, this new vision of a fashion-literate PM's wife is radical. As is the middle-class girl who's resuscitating The Firm. Sam Cam can afford to take sartorial risks - she won't always be in public life. The Duchess of Cambridge is in it for the duration. In their divergent ways, both women are emblems of modern Britain: image-conscious, far more outward looking than previously, subject to sometimes overwhelming levels of public scrutiny, not quite as at ease with itself as it would like to be. And still unsure where it should stand when it comes to displays of wealth and privilege. Read more from Lisa's column here