Homelessness and long-term unemployment aren't problems we generally associate with the royals, who've been born into a job for life and enjoy all the security, grand palaces and state dinners that come with privilege.

That might explain the surprise of London's commuters on Wednesday, when Prince William popped up on the city's streets, selling copies of the Big Issue magazine.

The publication covers homelessness and housing, and is sold by rough sleepers and other people below the breadline as a way of making a weekly income.

"My brother in law was in London today and saw a celebrity, so he took a photo at a distance," retired police officer Matthew Gardner wrote on LinkedIn. "The celebrity saw the 'covert surveillance' effort and crossed the road to investigate further."

He added that William whipped out a card machine when his relative told the prince he had no change to hand.

"What an honour to have a private moment with our future King who was humble and working quietly in the background, helping the most needy," Gardner said.

The pictures posted by Gardner and other Londoners quickly went viral, and became the best possible PR the royals could have hoped for since ... well, the weekend's triumphant jubilee celebrations.

The contrast between these two events gave the images added potency.

On Sunday, we saw Prince William on the balcony of Buckingham Palace next to his grandmother, waving regally to a sea of subjects. And the next week, he was incognito on the streets, selling magazines for loose change.

The importance of the cause to William was perhaps rooted in childhood.

His mother, Diana, took him and brother Harry to homeless shelters after dark when they were young.

It was an issue close to the princess' heart and one to which William has also committed as part of her legacy. I

n 2009, he spent a night sleeping rough on London's streets to experience the realities of homelessness for himself.

These experiences would have provided valuable insight into the problem of homelessness, which has been rising in recent years. More than 274,000 people are homeless in England, including 126,000 children, according to research published by charity Shelter in December last year.

Some 2,700 people are sleeping rough on any given night, the charity says, while many families are in unsuitable temporary accommodation.

"We're flooded with calls from families and people of all ages who are homeless or on the verge of losing their home," Polly Neate, chief executive of Shelter, said when the data was released.

The cost-of-living crisis hitting British families at the moment could worsen the situation.

And by spending time in the "Real world" after a weekend of pomp and ceremony at Buckingham Palace, William showed he can remain in touch with the people who will one day look to him as their monarch.

William's undercover volunteering will have provided good PR, but that may not have been the intention.

For the second in line to the throne, the experience was probably about making himself a better future King.

Story By | Max Foster & Rob Picheta