When Princess Beatrice and husband Edo Mapelli Mozzi went out for dinner with friends in Notting Hill, one thing appeared rather out of place.

It was not the princess's electric-blue dress and matching heels. Nor was it White House crisis manager Dustee Jenkins's flowing red piece with poppy motifs.

No, it was instead Edo’s scruffy white trainers, worn to the Michelin-starred Dorian restaurant for last weekend's visit.

To the untrained eye, they are simply a battered pair of pumps. But to anyone familiar with the strange sartorial rules of the British aristocracy, they are the ultimate declaration of wealth.

For while the aspirational middle-classes often dress to appear expensive, those born into Britain’s upper echelons have long understood that the greatest luxury is being able to look as though you have made minimal effort.

And nowhere is this more apparent than within the Royal Family.

Take Princess Anne. Here is a woman who could wear anything she wanted, glittering tiara and all. Yet the 76-year-old is more likely to be seen in a waxed jacket, sturdy boots and old trousers than anything resembling high fashion.

Anne’s wardrobe is unfailingly practical: her clothes are worn, reworn and built for the British countryside. 

And there is no need to signal expense or luxury because Anne is, of course, the Princess Royal. Her social status is more than assured.

Prince William has inherited much the same code. Away from black-tie events and ceremonial occasions, he is seldom seen in anything that screams luxury.

His uniform is instead classic posh man: the flat cap, faded Barbour, open-necked shirt and jeans or chinos.

Critics have compared his style with that of his more flamboyant father, who is often decked out in Savile Row tailoring and a flourish of pocket square. But look closer and even the King has his own aristocratic way of flouting the rules.

Who could forget his 2023 visit to the Brick Lane Mosque in east London when, after removing his shoes before entering the place of worship, an eagle-eyed photographer captured a close-up revealing a hole in his sock?

Royal watchers took it as the ultimate expression of upper-class nonchalance. The King does not need a pristine pair of £110 Gucci socks. As the linchpin of the British class system, there is no one he needs to impress.

And then there is Zara Tindall, whose wardrobe may be the ultimate demonstration of quiet wealth.

Anne's daughter can turn up at a race meeting in a £1,000 dress. But she can just as easily be spotted dressed down in jeans, riding boots and an anorak, her hair scraped into a messy ponytail.

And the message is crystal clear. It is not: ‘I cannot afford designer clothes.’ It is: ‘I have no need to prove that I can.’

Fashion expert and commentator Caroline Baxter says: ‘For years, fashion has told us that the ultimate expression of wealth is to make it look as though you don’t have any. 

'Stealth wealth: the £2,000 coat, the cashmere knit with no logo, the Cartier watch hidden beneath a cuff, the diamond studs. The richer you are, the less you’re supposed to look as though you care about being rich.’

Could this, then, be the great paradox of posh dressing? The more secure you feel in your social position, the less need there is to advertise it through your clothes.

Of course, there is a difference between looking cheap and looking casually expensive. The trick lies in the details.

A £50 fake Barbour worn by someone desperate to look ‘country’ can look very different from an authentic, mud-splattered Barbour worn by someone who actually lives and breathes the countryside.

Likewise, a pair of battered trainers can be simply battered trainers. Or they can be the footwear of a man who has just come from the family estate, the polo field or a Michelin-starred restaurant and simply couldn’t care less what his shoes look like.

So how can you tell whether someone is genuinely posh – or merely dressed to look it? The answer may lie not in the price tag, but all those things that they are conspicuously not wearing.