“I always tell my kids, the more you mess about, the more you find out,” she said, using a fruitier term than “mess.”

“Quite quickly, I realized that work was something I needed,” she said, “and I think I had a sense it was actually going to be within fashion,” even if she knew she didn’t want to go back to what she had done. In most big houses, designers’ jobs end at the runway. They don’t oversee the ad campaigns or the merchandising or the store design. Ms. Philo wanted to have fingers in all of that. Even if independence and a start-up meant not flying first class or having a driver or lots of orchids in the office.

“Fundamentally, that is not the stuff that makes me happy,” Ms. Philo said. The stuff that makes her happy involves baking, galleries, riding, clubbing, her family, her friends. She said she is constantly “walking the tightrope” between ensuring downtime and discovering inspiration. “Once she knows she can trust you, there are no barriers,” Ms. Rogers said.

After Ms. Rogers’s husband, the architect Richard Rogers, fell during a trip to Mexico and was in the hospital for months, Ms. Philo came over for breakfast one day wearing a big gray tweed coat Ms. Rogers admired. “She just took it off and gave it to me,” Ms. Rogers said, and refused to take it back. “It’s kept me safe and warm since.”

Edward Enninful, the former editor of British Vogue, who has been friends with Ms. Philo since they were kids in West London, said he used to bug her endlessly about when she would make men’s wear. “I always expected I would have to buy one of her women’s coats and get it tailored,” he said.

Then, just before the Fashion Awards in London last year, she presented him with a gray double-breasted suit, “just because she wanted me to feel good about myself,” he said. “I always wear black. I had never worn gray in my life, but I trusted her. It was very liberating.”