Saying “pardon” instead of “excuse me.” Calling your cook a chef. Pitching up on a grouse moor in brand-new shooting gear. In the 1950s, when the writer and bright young thing Nancy Mitford popularized the phrase “non-u” (“u” standing for “upper class”), this is what she was talking about. Although, as Lady Isabella Cawdor, one half of the new design duo at Holland & Holland, the famous gunsmiths and purveyors of hunting attire, cautiously allows, “Very, very stylish people can always get away with wearing everything brand-new. I mean, everyone’s got a new suit at some point, haven’t they?”

Cawdor and the famed model Stella Tennant, the best friend who roped her into the gig in the first place, are sitting in an annex of Holland & Holland’s newly refurbished headquarters in the Mayfair neighborhood of London. A riot of delphiniums and hydrangeas tastefully festooning the outside railings is evidence of the cocktail party held the night before to launch the pair’s debut ready-to-wear collection for the brand. It was, to be sure, a very “u” affair, where gin sours and Scotch quail eggs were served to guests like Rifat Ozbek, Jasmine Guinness, and “Bunter” Worcester, aka the future Duke of Beaufort. (Later on, Lady Jean Campbell, Cawdor’s daughter, brought her pal Pharrell Williams to the store for a peek.) The vibe of the place is a mixture of old-school cool and Brit eccentric. There are poems etched onto the wall by the artist Hugo Guinness, a creative collaborator on Wes Anderson’s film The Grand Budapest Hotel and a scion of the famous Anglo-Irish brewing family. The gilded twigs and glass domes full of butterflies and the dramatic console cast from the mold of a windswept hawthorn hedge are all by Tennant & Tennant, the interiors company Stella founded in 2011 with her sister Issy.

Tennant, 45, dressed in slouchy Holland & Holland Lovat-tweed herringbone “trizers,” as she pronounces them, and a lilac silk houndstooth shirt, is a perfect ambassador for the brand. She was brought up on a working sheep farm in Scotland and now lives near Berwick-upon-Tweed, in the Scottish Borders, with her husband, the photographer-turned-osteopath David Lasnet, and their four children. Sparkling on her wrist are some of the “Mitford diamonds” she inherited from her late grandmother the Dowager Duchess of Devonshire, whose sister Nancy was Tennant’s great-aunt. “Not bad, are they?” Tennant says with a giggle. “I figure they’re safer on my wrist than anywhere else.” Long gone is the bull ring Tennant used to wear in her septum, the one she refused to take out for that seminal 1993 “London Babes” shoot for British Vogue (remember the one, photographed by Steven Meisel, styled by the late Isabella Blow, and featuring those other “non-models” Bella Freud, Honor Fraser, and Plum Sykes?). Gone too, much more recently, is her trademark cap of dyed black hair. “It was great for modeling because you get these sharp images, but God, my husband hated it,” Tennant says. Cawdor, 49, a steely latter-day Lady Mary, with her English-rose complexion and cool, bred-in-the-bone poise, completes the perfect town-and-country picture. She is wearing a navy twill skirt and a field jacket (both Holland & Holland, naturally) with nice deep pockets—“so cheap, somehow, shallow pockets,” she says, ­grimacing. A fashion consultant and former Vogue stylist, Cawdor splits her time between Notting Hill and the 42,000-acre shooting estate in the Scottish Highlands that she maintains with her husband, Colin Robert Vaughan Campbell, the 7th Earl of Cawdor and the 25th Thane of Cawdor (Macbeth, FYI, was the second), and their four children. She was born Lady Isabella Rachel Stanhope; her father is the Earl of Harrington, and she grew up on the family estate at Ballingarry, County Limerick, in Ireland. Cawdor describes her childhood as “quite feral, really.” She and her brother were largely left to make houses out of bales in the hay barn and “flourish like weeds.” Nonetheless, Cawdor was always deeply interested in fashion, playing dress-up in her grandmother’s clothes and flicking through the pages of fashion magazines.

The designer Lady Isabella Cawdor wears all Holland & Holland. Cawdor’s son, James Chester Campbell, Viscount Emlyn, wears a Holland & Holland sweater; his own jeans and shoes. In the background is their friend’s jack russell, barry.

Photographs by Tim Walker, Styled by Bay Garnett

Cawdor met Tennant in the early ’90s on a fashion shoot in Arizona, when Tennant was just out of art school and newly discovered as a model. They immediately clicked and stayed friends, even as they got married and started families. This Christmas, they are all going to Kenya together. “Oh, our lives are extremely intertwined,” Tennant says. “Our children are adopted cousins, our dogs get on terribly well—I’ve even got a Labrador that came from Cawdor.” It was a no-brainer for Tennant to turn to her best friend when she got the call from Holland & Holland. “I said no at first, but they were very convincing, so I said I could only possibly do it if Isabella did it with me, not thinking she’d be up for it because she was too busy.” She turns to Cawdor. “Then you said yes, and off we started on this journey together.”

Founded in 1835 by Harris Holland (his original work desk sits in the London boutique’s entryway, inkwells and all) with two royal warrants, Holland & Holland is where posh folk have always turned for their shotguns, shooting gear, and accessories. In 1989 the company was discreetly purchased by the Wertheimer family (who also owns Chanel), but you’d never have known it from its somewhat fusty reputation—until now, that is. Tennant and Cawdor’s brief is to make it appealing to a more style-savvy crowd. Their first collection includes velvety rabbit gilets tied with grosgrain ribbon, luxe tweed plus-eights (literally, trousers that extend eight inches below the knee), leather walking shoes with fold-back tongues to stop the laces catching in the heather (perfect with scrunched-down shooting socks), cashmere sweaters in mustard and tangerine (made by Barrie, the Hawick-based mill), and, of course, kilts, with six-inch pleats—which, as Cawdor points out, are made using almost double the fabric of one “you might buy from a fashion brand or a tourist shop.” The clothes are just the thing for walking the dogs out on the glen—or high tea at the Ritz. “Who has time for two separate wardrobes?” Cawdor asks rhetorically, then shrugs.

Plenty of women concur. “Occasionally, designers do a great take on the outdoors—like, I’ll see someone on a shooting weekend looking fabulous and I’ll ask where the piece is from, and they’ll say Chloé 1992—but it’s very rare,” says Amanda Brooks, the writer and former fashion director for Barneys New York, who lives in a rambling Cotswolds farmhouse in Oxfordshire. “Most designers can’t resist adding an unnecessary epaulet or pocket. What’s great about this collection is how it is so restrained, pared down to the minimum, no bells and whistles, but with these fantastic, unexpected proportions. The clothes are really chic, but without shouting ‘fashion.’ ”

For Tennant, it was important to design functional clothes that still allow women to show off their figures. “People get frightened of wearing a kilt because of the amount of fabric. They think it’s going to make them look bulky, but actually, it’s the reverse: A proper one is flat on the tummy and then goes into a full skirt, which moves really beautifully and is quite flattering.” (She’s right. I’m not a six-foot beanpole, but in no way did the Holland & Holland kilt I tried on make me feel like a zeppelin. Quite the opposite, in fact.)

Cawdor’s daughter, the model Lady Jean Campbell, and Tennant both wear all Holland & Holland. campbell holds freud, tennant’s whippet. Hair by Philippe Tholimet; makeup by Lucy Bridge. Production by Jeff Delich; Printing: Graeme Bulcraig at Touch Digital; Photography assistants: Sarah Lloyd AND Tony Ivanov; Fashion assistant: Beatriz de Cossio; Special Thanks to Emma Dalzell.

Photographs by Tim Walker, Styled by Bay Garnett

The pieces, they hope, will be lovingly worn for years and years. “My mother [the artist Lady Emma Tennant] talks about buildings being in a pleasing state of decay—well, the same applies to clothes,” Tennant says, turning to Cawdor. “For example, Isabella’s got the most fabulous black silk shirt, haven’t you? Which is worn on the collars and cuffs, and everyone thinks it’s some designer shirt.…” “Well, it’s only a recent thing that a woman doesn’t have a tailor,” Cawdor rejoins. “When I was growing up in Limerick, there was this local tailor, Mr. Fraser, who boasted about how he knew everybody’s bust, waist, and hip measurements. He made me this great pair of black trousers when I was 17, and I’ve still got them.”

In between working on the line at their studio in Paris and shuttling back and forth to Milan for fittings, both women insist on spending as much time as they can in Scotland. Tennant, at an 18th-century eight-bedroom Georgian manse with its own stream (“I thought I’d be done with modeling when I decided to move full-time to Scotland, and then United went and laid on a lovely direct flight from Edinburgh to New York, and, well…job done!”); Cawdor, at Carnoch, a mid-19th-century lodge overlooking the River Findhorn. (The estate’s actual castle, which dates back to the 14th century and has a drawbridge and moat, is currently inhabited by Colin’s stepmother, Angelika, the Dowager Countess Cawdor.) Stella and clan are frequent visitors. They all shoot, gallop horses across the beach, canoe to a bothy where mussels are collected and barbecued, pick chanterelles in the glen, and fish for salmon and langoustines in the sea lochs. “We’re an hour away from a loaf of bread,” Cawdor says. “So online groceries kind of changed my life. Though I prefer cooking when you don’t have to actually buy the food.”

But back to those scrumptious clothes—namely, the shaved-weasel skirt that Tennant wore to great effect at their launch party. It reminded me, in a way, of Méret Oppenheim’s surrealist 1936 artwork Le Déjeuner en Fourrure. “Oh, you mean the fur teacup?” Tennant asks, beaming. “Actually, the skirt was based on this picture we have on our mood board of a Sevillian lady wearing this suede shirt and skirt that almost looks like an apron.”

“But the point is that it’s warm,” Cawdor says firmly. “You can sit on it on the ground. You’re not going to get a wet backside or chilly kidneys or purple lips. After all, nobody looks attractive when they are freezing cold, do they?”

Photographs by Tim Walker, Styled by Bay Garnett