If In-N-Out Burger is seriously testing the waters in London, it’s got to be happy with the results.

On Wednesday, the California-based fast-food chain with a cult following opened a pop-up store in the U.K. capital, and the temporary location quickly sold out of hamburgers, cheeseburgers and double-doubles.

The one-day pop-up — which had been slated to offer its fare from 11 a.m. to 3 p.m. London time — had to start turning people away by noon, according to Eric Billings, In-N-Out’s manager of special foreign events. He said a line started to form at about 8 a.m., and customers began to get wristbands at 11 a.m. that let them claim a burger later.

Billings declined to give the number of burgers sold, but the video below of the line outside shows how In-N-Out drew a big crowd. The pop-up was in the Swiss Cottage neighborhood, a less glamorous part of the capital.

Some Londoners who waited in line and secured burgers essentially licked their lips when asked about maybe one day having a permanent In-N-Out location in their city.

“I think In-N-Out could definitely work in the U.K.,” said Nico Amurao, a 32-year-old Londoner. He said the city’s current hamburger offerings aren’t that strong, adding that Wednesday's sizable turnout showed there is clear interest.

The timing would be good for In-N-Out to move in, with higher-end burger joints popular right now, said Ryan Murray, a 42-year-old Londoner. Murray said he has had better burgers than the one he got Wednesday from In-N-Out, but he ranks the chain’s offerings on the high end for fast food, and simultaneously at the “low end of the gourmet burger phenomenon.”

The burger business is a fertile field in London, with U.S. chains like Five Guys and Shake Shack SHAK, +1.16% operating in the city, competing against British chains like Gourmet Burger Kitchen and Byron. And there are, of course, plenty of Burger King and McDonald’s MCD, locations (and Mickey D’s CEO is a Brit nowadays).

In-N-Out did not signal that it’s about to set up shop for real in London, but the chain did hint that this is a possibility.

“We have done events like this before in other countries, and they are just one part of our efforts to promote and expand our brand, as well as determine the best way to continue reaching out to customers around the world,” said Carl Van Fleet, In-N-Out’s vice president for planning and development, in a prepared statement.

“These events also help to protect the In-N-Out Burger brand in important regions like England and Southeast Asia,” he added. “We do not have any immediate plans to open permanent restaurants there, but these special events will help us make future decisions.”

Not all who waited in line were enthusiastic about the possibility of In-N-Out coming to London for more than a day.

If the chain starts opening permanent locations outside its core market in the western U.S., that could take away from its cachet, said Amanda Do, a 21-year-old UCLA undergrad currently studying in London.

“I think it’s the novelty that makes it special,” she said.

Read more:McDonald’s tests premium burger in London — and it’s no Big Mac

And see:Battle of the burgers: Shake Shack vs. In-N-Out

This story was first published on Sept. 21, 2016.