She was fooled, for a flash, then realized she was looking at professional actors expertly dressed and made up to look like three of the most famous people in the world.
"They look so in love," she said of the couple playing William and Kate, who in real life plan to marry April 29 at Westminster Abbey. "I didn't expect to see it."
The imitators (fraudsters, if you like) were promoting a new book by Alison Jackson that pokes fun at the royals in a series of racy photos. The book is one of a series of spoofs, including elaborate newspaper and magazine productions, capitalizing on the fascination -- and occasional fatigue -- with the escalating blitz of publicity leading up to the royal wedding.
April Fool's Day on Friday provided a perfect launchpad for some of the William-and-Middleton hoaxes, including an editorial in The Guardian newspaper renouncing its earlier anti-monarchist stance and announcing a full-throated, faked-for-the-day endorsement of the royals.
The paper said it would be recalling its correspondents from some "less newsworthy" places like north Africa so they could provide unparalleled coverage of wedding preparations, including a 24-hour-a-day, minute-by-minute blog.
"It is time to put away the cynicism, and get out the union jacks," the newspaper announced in mock sincerity.
That's tame compared to the popular Sun tabloid, which has reported in recent days that aliens are planning to attend the wedding in UFOs, and The Daily Mail, which showed a lookalike Middleton model shopping for baby clothes.
Some newspapers have devoted substantial portions of their Sunday magazines in recent weeks to satirizing the wedding, and the nation's obsession with it, with The Times spending freely to have a columnist dressed up as Middleton for a text and story feature called "Caitlin Moran's big fake royal wedding."
It gave the acerbic writer a chance to make fun of the royals and all the over-the-top wedding preparations -- while still attracting readers who can't get enough of all things Middleton.
Why make fun of the royals at what's supposed to be a time of national celebration?
Moran said the satires reflect a sense that the actual wedding will be so well scripted that it will inevitably be a bit dull.
"When the real wedding happens it will be quite boring," she said, implying that the bogus weddings -- like the one between the lookalikes who commandeered a church Friday afternoon -- would actually be more spontaneous.
She said it is possible more money will be spent on this wedding than on any other nuptials in the world, making it difficult to take the whole thing seriously.
"If you're going to have a monarchy, you have to make fun of them when they're going to get married," she said. "Otherwise there's no point."
After all, irreverence is part of British tradition -- even the queen and her corgis are fair game, as long as it's done with an underlying hint of respect.
"It's fun how the English do this," said Iris Bitzigeio of Germany as the fake royal entourage went by in a very real horse-drawn carriage that was adding to the midday traffic jam in Piccadilly.
One visitor from Australia, 27-year-old Krystal Macmillan, was completely taken in by the stunt, quickly ending her cellphone conversation when she saw the bogus royals go by. She realized fairly quickly she had been too gullible.
"I'm not used to queens and princesses running around," she said.
Londoner Veneena Alacrone, 19, was not fooled by the costumes -- any true Brit can tell the real queen from a fake -- but she got a kick out of the stunt.
"I knew right away, I'm not stupid," she said. "But I think it's funny. I think the wedding's really nice. It's pulled everyone together, everyone's talking about it."
She and her friends are planning a bachelorette party -- called a "hen do" in British slang -- in Middleton's honor. And they're happy about another aspect of the wedding: The fact that April 29 been declared a national holiday.
"We get a day off school," she said.
Aaron Edwards and Kali Borovic contributed to this report.