Transcript for How Meghan Markle's 'California casual' style could impact British fashion: Part 4

Announcer: Now from Buckingham palace, we continue with Deborah Roberts. I'm here in front of Buckingham palace where a 15-year-old Meghan Markle was photographed as a tourist never imagining she would join the royal family. Now she's being photographed on the world's stage and bringing a whole new dazzle to the Windsor wardrobe. ?????? Meghan Markle, first self-madr in the royal family. So she's very definite in her style. Reporter: Meghan Markle has known from the beginning what looks good. She's been getting it right on red carpets for years. But when Diana arrived on the scene at just 19, she was unschooled in the worldly ways. Careful. She said to me, you know she had no smart dresses, no smart clothes and she very quickly had to get a wardrobe together. Reporter: What a wardrobe that was. Prince Charles himself has taken a publicity backseat to his wife's trendsetting clothes. The DI look is described as exciting, although it is in essence traditional and classic. Whatever it is it is selling like hotcakes. As the years went on, after her divorce, as Diana grew more confident in her personal life, she seemed to grow bolder in her fashion choices. You started seeing her in Versace. Some of those fabulously sexy outfits. I think everybody was amazed at Diana's beauty. She became this style icon. Her choices of those dresses kind of gave us a new way of defining what royal glamour could be. Reporter: Amanda Wakeley is one of the few designers who has dressed both Diana and Meghan. How would you compare her style and Diana's style? I think Diana really, really understood her own style and I think Meghan really, in the same way, really owns her look, she knows what works on her. Reporter: For Meghan, her royal glamour starts with California casual. Just ask her longtime friend and makeup artist Daniel martin. What Meghan's most comfortable in is a cashmere sweater and skinny jeans. Meghan's personal style is very chic, very sophisticated, but still very understated. You know what nobody likes, nobody like a show-off. Reporter: She credits her seven years starring as Rachel Zane on "Suits" with her own style evolution, as she told "The outnet." Everything that I used to wear was a bit more chill and now, because of our exposure to these amazing designers, it's kind of amazing how my style has become a little more polished and adapted to that aesthetic. Reporter: Vanessa kingori, British vogue publishing director says Meghan's breezy style is a breath of fresh air. I think a lot of people are saying, "Finally," you know, she looks like me. I think it's fabulous she's of mixed heritage. People can see their daughter, themselves, their friends in her. What happens to a designer when Meghan Markle wears his or her clothing? It's really incredible. There's this anticipation that Meghan's fashion choices will add over a billion pounds to the British economy. Wow. Yes. There's this wonderful story about her carrying this tote from strathberry yeah. It was sold out in 11 minutes. Like that. For those small brands, it's absolutely transformative. Reporter: It's not just designers cashing in. Wait, who is that? For just a second, did you think it was the real person? Do you want to take a picture? Emma Kaler is a teenage Meghan lookalike making appearances as a way to save money for college. When's your next gig? On Monday, I'll be modeling the dress she's going to wear on Saturday. That should be really good so, I'm hoping it's something lovely. I'm sure it will. Reporter: In mere hours, the real Meghan Markle will make her biggest fashion statement yet as she steps into history. In July of 1981, it was Diana who wowed the world, including ABC's Peter Jennings. And here, our first glorious view of the bride. That was the flounciest, most over the top really, wedding dress that you could probably have. Puffy sleeves. Huge puffy sleeves. It was ruffles upon ruffles, upon ruffles with a train of ruffles. Reporter: 10,000 hand-embroidered mother-of-pearl sequins and a 25 foot train. This is magnificent. I've never seen a train like this. Have you? I've never seen a royal wedding like this. Reporter: Those ruffles and puffy sleeves fluttered their way right into the mainstream 80s fashion. Diana has single and laces from obscurity. Reporter: Now it's Meghan's turn. At this point, every single designer in the country has been accused of working on this dress in secret. Who's your best guess? The three top brands I would say. One is erdam, he's a Canadian designer who lives and works in Britain. Much like Meghan, who lived in Canada and now is here. There's also discussion around Christopher Bailey having just retired from burberry after a great reign. Reporter: Meghan's own mom adding heat to this theory when she was spotted leaving for London with a burberry garment bag. And Ralph and Russo, also, she's worn them in the past. Reporter: We've seen her before in a wedding gown. She walked down the aisle in this dress for her final episode on suits. You're the husband I've always wanted, I can't wait to begin our adventure together. Reporter: And at her first wedding to Trevor Engelson. She wore this simple sheath. Meghan is getting married in a stunning gothic chapel.s to have a grand dress to match the venue. That's the most important thing. So while she, perhaps, might instinctively want to go for something fairly simple, she is going to have to take into consideration this incredible atmosphere, so it will be a little more elaborate than perhaps she'd like. It's very possible that Meghan could do something like take one of Diana's iconic pieces of jewelry, and turn it in to a tiara. One of kind tiaras have been made before, so that would be a beautiful thing to do.

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