She’s the designer who can make grownup professional females with wardrobes full of sensible clothes dream of First Communion dresses—and dare to wear white after their wedding day.

Daniella Helayel, a diminutive powerhouse of British fashion, is on a summer mission to make us All White on the night—or even in the day.

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Credited with turning Prince William’s girlfriend, Kate Middleton, from a safe-looking Chelsea girl into a confident and quirky dresser, she’s sponsored a transformation royal watchers have dubbed “Post-Diana NuGlam.”

Princess Beatrice, for the next generation, also parties forth in Issa.

Helayel’s classic frocks, with their flattering contours and easy fabrics, have become a staple for women who don’t know where the iron is. That must surely include Madonna, who wore an Issa dress to soften her look, when she was pictured recently in Africa, on her quest to adopt another baby.

I encountered Issa clothes at a Vogue charity sale, as Jemima Khan stretched out a languid hand to grab the same bargain. “Please, no,” I thought. “You have a trust fund and (sometimes) Hugh Grant as a boyfriend. My need is so much greater than yours.”

Fortunately, Ms. Khan was diverted: I grabbed a frock—and never looked back: Issa converts are for life.

On Thursday, Helayel launched her next cruise collection in New York.

Perhaps it’s her Brazilian heritage that makes her comfortable with strong southern colors and contrasts—strong black, royal blue, and pinky-red prints, trailing seasonal butterfly patterns, and satisfying poncho-based capes in pleated fabrics, to swirl dramatically over jeans or short skirts.

In the doldrums of economic gloom, she’s also cheered us by rehabilitating the look we thought we’d never dare to wear beyond bridal day: a white dress, in her trademark Lucky style.

Issa struck gold with the Lucky dress in black, red, and denim last year. This year, she’s brought it back for summer in a range of virginal white—with minutely differing detail, tiny pintucks, and delicate fabric.

It’s girlish, but thoroughly wearable by women whose first date lies far in the past.

Laura Craik, the London Evening Standard’s judicious fashion editor, praised it last year in the Daily Mail as the “short but-not-too-short, sweet-but-not-too-sweet dress” and dubbed it a “dress of the season.”

“Sometimes a girl doesn’t want an all-singing, all-dancing dress in the latest hot color or print,” she wrote. “Sometimes, a girl just wants a dress that will make her feel brilliant.”

Amen to that.

I meet Helayel at a modest workshop in the industrial quarter of London’s Chelsea, where she does her “doodles,” as she calls the distinctive designs that have seen her company challenge some of the bigger and better funded names in fashion.

She’s in a fabulous snit the day I visit, because a delivery for autumn-winter hasn’t got the detailing right and doesn’t stint on the abuse.

A patient assistant soothes and makes long notes of people to be phoned and told, in certain forceful terms, that this print is blurred, or that stitching too large. “I’m sorry, but I have to deal with this, because it just won’t do,” she says after one lengthy peroration.

By now she’s slumped on a faux leopard sofa, and stabbing at a BlackBerry.

I feel a surge of pity for whomever is on the other end.

Lucky, she tells me, was inspired by a vintage baby’s dress—with puff sleeves adjusted to a scale that won’t make the modern wearer feel like Anne of Green Gables.

Helayel hadn’t even intended the white Lucky to sell out: “I wore my white one to almost every party last summer, and women would always come up to me and ask me where they could buy it,” she says. (Her top tip: If you think you really cannot wear white without some help, throw a navy blazer over it—it works for everyone.)

Production was stepped up and the design refined and diversified. Warming to her white-hot theme, she tempted the (famously fussy) Naomi Campbell into a cleverly gathered, creamy-white evening gown for her summer show.

Alice Dellal has worn it in red; Scarlett Johansson has a backless version. It’s good if you’re curvy, because it gives you a gently bosomed look, without barmaid cleavage.

Helayel’s aim is to create “Issa for all”—her designs work for women of different shapes and ages: “My granny wears my clothes, too, which makes me feel really good.”

No wonder clever Daniella got so Lucky.

Anne McElvoy writes the main weekly political column on domestic and international issues at the London Evening Standard. She is a regular broadcaster, appearing on Newsnight and Question Time, among other programs. She has published two books on Germany, The Saddled Cow: East Germany’s Life & Legacy in 1992 and she was co-author of the memoirs of the spymaster Markus Wolf, Man Without a Face.