Almost two decades ago a pair of fresh-faced British sketch comedians armed with a good idea and an able director with a cache of film stock made a charming short film called “The One and Only Herb McGwyer Plays Wallis Island.” The 25-minute outing won a prize at the 2008 Edinburgh Film Festival, was nominated for a BAFTA and announced the arrival of Tim Key and Tom Basden. The two spent the intervening years turning their penchant for absurdist humor into sketch comedy shows, radio episodes, stand up poetry tours and sidekick roles in film and television.

But they never returned to Wallis island.

Until now. Older, grayer and maybe a little wiser, the friends, onetime roommates and longtime collaborators have expanded their initial concept into a feature film, “The Ballad of Wallis Island.” The film, which ruminates on love and loss, revolves around a musician who is hired by a two-time lottery winner to perform a private gig on an isolated island. It feels like it could have been created only by filmmakers with a little road beneath their feet.

“I don’t really regret us not making it 17 years ago, because we just might not have been able to do it right,” said Key, who wrote the script with Basden and plays the rich eccentric, Charles Heath, who prattles through conversations with a stream of nonsensical puns. “I think when we came back to it, we were more ready to make a decent fist of it.”

The original director, James Griffiths, returns, and the main conceit of the short remains: The musician, Herb McGwyer (Basden), arrives at the harborless, fictional Wallis Island (portrayed in and around Carmarthenshire, Wales) to perform a concert for his eager audience of one (Key’s Heath). To build out the story, Basden and Key introduce Nell Mortimer, played by Carey Mulligan, McGwyer’s former singing partner and lover from their short-lived duo McGwyer Mortimer. When she shows up on the island unbeknown to McGwyer — whose solo career hasn’t gone as planned — the film gains its emotional heft.

“You get a window into what they were like when they were young and into the way that life has or hasn’t messed with their expectations as young people in the music industry, and as a young couple in love,” said Basden, who also wrote the songs for the film. “When you engage with that meaningfully, I think you’re always going to end up having to write about the loss, the heartbreak and the regret that goes with relationships in your 20s.”

The result is a charming crowd-pleaser that evokes John Carney’s music-driven films such as “Once” and “Sing Street,” yet is careful not to strike any of those chords too fiercely. “Wallis Island” has been making the rounds of the U.S. film festivals, delighting audiences at both Sundance and SXSW before being released in New York and Los Angeles this weekend. (It will go wide on April 18.)

We caught up with the stars at SXSW the night after the film screened and Basden took to the stage to play a song from the film. The moment reminded Mulligan why she isn’t a singer in real life, but only plays one in films. “I don’t know how you did that,” Mulligan said to Basden the next morning over a plate of fruit, harking back to her press tour for “Inside Llewyn Davis,” when she was put onstage to sing alongside Patti Smith, Gillian Welch and a handful of other musical professionals. “It was the scariest thing I’ve ever done, above anything else. It’s so exposing, opening your mouth and trying to make melodic sounds in front of people.”

In contrast, Basden, now 45, has been doing it for decades. He and Key, 48, first met as members of a Cambridge University improv group, the Footlights. Basden was a student. Key pretended to be one. (His imagined area of study: an advanced degree in the Russian novelist Nikolai Gogol.) That work turned into a sketch comedy group called the Cowards, where they wrote and performed skits. The two made three short films together and regularly appeared in each other’s work.

In one live stand-up pop-up called “Freeze,” Key played the bully to Basden’s earnest singer-songwriter character, cutting him off as he played and mocking his singing. “It was a very good dynamic, because I was really unreasonable with him and quite aggressive, and the audience could see that he was really talented,” Key said.

In his bathroom, Basden has hung a framed note from a concerned audience member, urging him to part with Key. “He’s really bad and he’s so mean to you,” the letter reads.

Key and Basden find themselves becoming rather emotional as they tour around the U.S. with “Wallis Island,” moved that the film has resonated with American audiences.

“It does feel quite dreamlike to be able to make this feature,” said Key, best known to Anglophiles as the sidekick to Steve Coogan’s inept broadcaster character Alan Partridge. “Tom and I enjoy doing things together, but this was sort of the pinnacle, something to aim for,” he added. “So we suddenly have it all — all at once — it’s quite overwhelming.”

Part of the emotion is based on just how long it took to regroup on a feature film. Key, Basden and Griffiths had spent years developing a script, only to have it die in development, a process Griffiths refers to as, “a long walk up a windy beach to a cafe that’s closed.” He added, “We all felt there was a potential to make something together again, and we saw it slip through our hands a little bit.”

Mulligan, for one, was able to help make “Wallis Island” happen, courtesy of her husband, Marcus Mumford, the lead singer of the band Mumford & Sons. Mumford, says Mulligan, is an “obsessive” fan of Key’s Late Night Poetry show and reached out to Key via Instagram.

“It’s framed in my bathroom,” Key joked.

Mulligan then followed up, asking Key if he’d emcee the couple’s event for the charity War Child (Mulligan is a global ambassador for the group, which aids children in war-torn countries). Key declined. “I’d have been absolutely petrified,” he said.

But he did hold onto Mulligan’s email and after they crafted the role of Nell, Mulligan stood at the top of their list for the part. Key reveled in telling his collaborators he had a direct line into their future — and much more famous — co-star. Until he realized what was at stake: “This was an enormous fork in the road, a massive loss of trust from both of them if they thought I was lying about Carey Mulligan,” he said with a laugh. “I think I spent more time on that email than the script for the feature. It was like a three-month project. It went through development.”