While his fellow New York billionaire friends are battling over the White House, American designer Ralph Lauren is leading a different kind of battle; against cancer with the opening of a new hospital in London.

Ralph Lauren has dressed presidents and princesses, first ladies and film stars. Hillary Clinton is a friend, so was Diana, Princess of Wales. He was at Nancy Reagan’s funeral; one of his sons is married to a Bush. Lauren’s American Dream includes everyone, whatever dynasty you come from. “It doesn’t matter who you are, I have something for you,” he says. “I make clothes for all kinds of people.”

So does he dress The Donald? There is a pause as one New York billionaire assesses his relationship with another. “I don’t know if he wears my clothes or not.” (Melania, the third Mrs Trump, definitely does wear Ralph Lauren, I discover later.) “He comes to my restaurant.” Trump has been seen at Lauren’s Polo Bar in Manhattan, but what does Lauren think of him? “Well, you know, he’s a colourful character,” says the softly spoken designer carefully. “I like him. I know him personally. He’s a nice guy in person, you know. [But] I like Hillary a lot. I think she is very well equipped to do this job. It’s amazing how the world wants change in some way but you want the right people who really know their jobs to do good jobs.”

Could Trump be president? “I don’t know,” says the man who has not built a $5.5 billion fortune by shocking people, either with clothes or opinions. “Hillary is trained for it. She has done the job. She knows what’s going on. If there is anyone that’s equipped she is equipped and I think that’s what you want. You don’t want to find yourself in four wars all at once.”

It’s been reported that several other American billionaires are preparing to plough millions into the Trump campaign but I think we can safely say that Lauren won’t be one of them. However, he doesn’t rule out the possibility that Trump could win the presidency. People are “laughing about this guy. Meanwhile this guy is getting popularity from very reputable people. So who knows?”

It is hard to imagine two more different billionaires than Ralph Lauren and Donald Trump. Where Trump is brash and divisive, Lauren is smooth and inclusive. Trump wants to make America great again; in Lauren’s world America is great, inhabited by people with beautiful hair and perfect skin, whether they are fulfilling a western fantasy in plaid and denim, or preppying-up the Ivy League college look in blazer and chinos.

Trump has had to deny stories that he had pursued Diana, Princess of Wales. He said he had met her only once. Lauren, whose clothes she wore, knew her rather better.

We are talking in Claridge’s, shortly before the opening of the Ralph Lauren Centre for Breast Cancer Research at the Royal Marsden Hospital in London. The centre was opened by the Duke of Cambridge, who is president of the hospital, a position held previously by his mother.

Lauren met the princess at a lunch and suggested they meet at Claridge’s when he was next in London. “She came with her lady-in-waiting, I came with my son and we met here. I was wearing jeans and she said: ‘Who do you know that you can be wearing jeans? They don’t allow it in here.’ I said: ‘I know you.’ ”

Lauren has designed gowns for Michelle Obama and his clothes are also worn by Robin Wright’s fictional first lady, Claire Underwood, in House of Cards. Plenty of designers, though, have brought their measuring tape to the White House. It’s much harder to get the Windsor Castle invitation, which Lauren received two years ago when he sponsored a fundraising dinner there for the Royal Marsden hosted by Prince William. “He has taken [the Royal Marsden role] and made it his own,” says Lauren. “I guess it’s important to him. He’s terrific. Very gentle, a very nice guy.”

Lauren’s decision to make a large donation to the centre is the latest chapter in an almost 30-year personal history of supporting cancer charities. It was instigated by his own illness and the death of a friend. In 1987 Lauren was diagnosed with a brain tumour. It was removed and turned out to have been benign.

“But still, they opened up my head. I have scars here and here. I was very successful, everything was going great and this comes up. Still to this day when I see someone who has that I, choke up. I really became aware of life and felt like I got off and I could help someone else get off.”

After his operation his friend Nina Hyde, the fashion editor of The Washington Post, told him she had breast cancer. “I said, ‘I want to help. What can I do for you?’ She introduced me to a doctor in Washington, a very good cancer specialist, and he said to me that breast cancer would be cured in our lifetime. That’s 30 years ago and I thought at that time that I was going to help cure it.” He laughs wryly. “If he had said we are going to cure it in 100 years I would maybe not have got involved.”

Hyde spent time with Lauren at the designer’s ranch in Colorado. “There, with the big sky, you are closer to God. I didn’t think she would die but she did.” He and Katharine Graham, the late owner of The Washington Post, founded a breast cancer centre in Hyde’s name at Georgetown University. Lauren has funded other cancer projects in the US and designed the first T-shirt for the Fashion Targets Breast Cancer campaign, which has been running for more than 20 years. He shrugs off his contribution. “You realise you have done nothing and there is a long way to go.”

Hillary Clinton and Ralph Lauren

Lauren was born Ralph Lifshitz to first-generation Jewish immigrants from Belarus. His father was an artist, “but he struggled and painted houses sometimes because he couldn’t make a living. Life was a little tough.”

He changed his name at 16. After working at Brooks Brothers he started designing ties and next year it will be 50 years since he started selling ties under the Polo label. He followed up with a full menswear line and expanded into womenswear and eventually homeware, watches, fragrances and a range of clothing labels to suit different pockets. He once said he wasn’t designing clothes, he was “creating a world”.

Although his polo-player logo is known all over the globe, he has only ever played “cowboy polo, out west, where they use brooms. I like sports and I couldn’t call it baseball or basketball and I thought: ‘Polo.’ Although Americans didn’t know what polo was really, people who go to polo matches are stylish people. So I thought the name was really good and then I added my name.”

He started with clothes for men and he still finds them easier to design. “Menswear is more subtle than womenswear. Men don’t want to be fashion freaks. They want to look good but they don’t live for fashion. Women are much more fickle. They don’t care what brand it is, they want it. Women are more adventuresome; they are more experimental with dresses and skirts and shapes.”

What he regards as timeless English style has always been a key influence. “I have always loved England because of the ‘non-fashion fashion’. English style was one of my inspirations because it was not about fashion. You wear a sweater when gardening and put a patch on it and it looks great.”

Ralph Lauren

He is encouraged by the sense of style he sees in London these days, after a period when he worried about us. “When I first came to London I loved English clothes and I was shocked when I saw one time a lot of flashy clothes that didn’t look like England. England was trying to look like something else. Now England looks like England.”

One of the things this boy from the Bronx is particularly proud of is that for several years he has been designing the outfits for officials at Wimbledon. “Wimbledon is beautiful, it is traditional. I am a traditionalist. It’s not that I am backwards, it’s just that I love heritage, I love longevity. That’s been my philosophy, but you constantly update so that you look contemporary. You have to move with the times and update but understand that the past also has a wonderful sensibility.”

He has a museum-quality collection of classic cars and waxes mechanical about British marques, including Jaguar, Aston Martin and an old Morgan he once owned. What about Rolls-Royce? “No, too flashy.” Then he adds quickly: “I love seeing the Queen in one.”

While he talks about heritage he doesn’t seem to dwell on his own. Of his parents he says: “They came from parts of Russia. I opened a few shops in Russia but I couldn’t connect exactly where they were from.” I had read that they were from Belarus. “You have more information than I do,” he says genially.

When I ask if he is astonished at how far he has come from his humble upbringing, he says matter-of-factly: “No, I don’t feel that way. I just keep going. I still work hard every day. I do the same thing I have always done. I’m not ready to stop. I like what I do.”

He concedes, however, that “I can’t do it for ever”. Last year he handed over the chief executive role to Stefan Larsson, who had been running the Gap-owned Old Navy. For now he will remain as executive chairman and chief creative officer. The challenge is to ensure that a brand that he embodies can one day survive without him. He says that Valentino and Givenchy have both retired and their companies live on.

Lauren has two sons and a daughter with Ricky, his wife of 51 years. She appears briefly at the start of the interview and is introduced as “my biker girl” because of the motorcycle jacket she is wearing. The whole family have always appeared in Ralph Lauren promotional material, playing at cowboys or hanging out on the beach. Ricky’s book about the family’s enviable summers in the Hamptons is on display in the store in Bond Street.

The Ricky bag is named after Lauren’s wife, but when I ask about her influence on the business he stresses that she has her own career as a therapist. His younger son, David — whose wife, Lauren Bush, niece of George W Bush — is a senior executive at the company. The older son, Andrew, is a film producer and his daughter, Dylan, owns what claims to be the biggest sweet shop in the world, in New York.

“I don’t know if they would run the company,” he says of his children, but he hopes they will be involved. Right now, he insists, “I feel like I’m 42. I don’t feel my age. Maybe I will.” He works out five days a week, but doesn’t find running easy because he has a pin in his ankle, the result of a car accident.

He works so hard at promoting the Ralph Lauren lifestyle. Does he get the chance to enjoy that lifestyle? “Yeah, I have a good life. I have no complaints. I bought this ranch and turned it into a working ranch. I would like to spend more time there but I don’t have that much.”

He is dressed, of course, head to toe in Ralph Lauren: jacket, shirt, tie, grey flannel trousers, even a Ralph Lauren watch and signet ring. I am wearing my best suit, which was handmade for my wedding 16 years ago. “I like what you are wearing. It’s low key, it’s timeless. Looks good on you,” he says.

I know he’s just being charming, but I’m happy to hear it. I don’t get complimented on my dress sense by one of the world’s most-famous menswear designers all that often. I respond by asking how influenced he was by John F Kennedy’s impeccable style. Very influenced, he says. That’s interesting, I say, because I was looking at some photographs of JFK the other day and he looked like . . .

“Like you,” cuts in Lauren, suavely, indicating my old-school tailoring. This is clearly absurd flattery, but, hey, this is Ralph Lauren. His whole world is based on the rest of us dreaming his dream.