Olivia Rodrigo sings about heartbreak but she's already chosen her wedding song

Before long, her Instagram page was filled with photos from Wimbledon and double-decker busses.

Headlining Glastonbury last summer, with Partridge at the side of the stage, she changed the lyrics from "I think I'm in love" to "'Cause I'm in love".

So when she sat down to write her third album, she thought it would be pretty straightforward.

"I really wanted to capture romantic joy and pleasure for the first time, because my last two albums were very heartbroken and really angsty," she says.

But one look at the title – You Seem Pretty Sad for a Girl So in Love – suggests paradise wasn't what it seemed.

"It's a love story that falls apart," she confirms.

"A time capsule of a relationship in a few years of my life."

It opens in a London pub, where Rodrigo is besotted by a boy who "looks like an angel on the walls of Versailles". He's so pretty she's paranoid she made him up. If he kisses her, she might drop dead.

By the second track, Stupid Song, they've hooked up; and she's so happy she can't write a sensible lyric.

"When you're really deeply in love, it feels like a song is so futile," she says. "It's really hard to capture in a way that feels palpable to others."

As the album progresses, however, doubts and anxieties creep in.

Everything hinges around track seven, Purple. The lyrics are happy, but the chords never resolve to the harmonic centre, creating a feeling of instability.

"Initially, it was a love song, and it was very sweet and saccharine," says Rodrigo. "And a few months after we wrote it, we revisited it and put new chords underneath it and tweaked some of the lyrics.

"So, yeah, it's definitely the part of the album where things start to sour and unravel."

A similar fate befell What's Wrong With Me – a duet with her musical hero, The Cure's Robert Smith.

It was originally written about missing someone so intensely she felt listless and depressed. Rodrigo revised the lyrics post break-up, after realising the relationship itself was the source of her sadness.

"I can't eat, I can't sleep / I think you're what's wrong with me."

She and Smith debuted the song last weekend at Spain's Primavera Festival. Backstage, the British star was full of praise for his protégé.

"She is genuinely fantastic, as a singer, as a songwriter, as a performer," he told BBC 6 Music. "I'm slightly in awe of how easy she finds it all."

It was their second appearance together, after duetting at Glastonbury last summer.

Universally acclaimed as the set of the weekend, the performance confirmed Rodrigo as a generational talent, equally at ease with lighters-aloft balladry, and rabble-rousing pop-punk antics.

Backstage, though, she was a bag of nerves.

"I remember having like a near anxiety attack in the bathroom, like, 'How am I going to do this? I don't know if I'm ready.'

"But something overcame me the second I stepped on stage and started singing. I felt, like, totally calm and so in my element.

"I'm not very spiritual or religious, but it's moments like that where I feel music is just so magical that you just can't really describe it."

It was also a performance powered by an impressive amount of dessert.

"I stayed at this [hotel] that had the best sticky toffee pudding, and I was like, 'You know what? I gotta do it'," she laughs.

Three bowlfuls were consumed before she took to the stage.

"If the toffee's really hot and the ice cream melts on top, it's really good."

Pudding isn't her only British culinary weakness.

"My favourite thing is dipping soldiers into an egg," she says. "I got really obsessed with it. My friends even bought me different egg cups from all over the world."

That's the charm of Olivia Rodrigo. She may have grown up in showbusiness, working on multiple Disney shows before launching her music career, but she's neither cosseted nor precious.

One of the reasons she loves the UK is that it gives her perspective.

"I feel so normal here, very adult. I can walk to the pub and meet friends. It's a city where spontaneity is really encouraged. People are very social here, in a way that they're not in Southern California."

And, for someone with 40 million Instagram followers, she seems resolutely unbothered by other people's opinions of how she should behave.

She's been outspoken about the curtailing of reproductive rights in the US, and the humanitarian crisis in Gaza.

Last year, she criticised the Trump administration for using her music in videos promoting ICE deportations, calling the government's policies "barbaric and cruel".

She chooses those battles carefully.

"I definitely try to be careful with my words," she says, "but simultaneously, the women I looked up to when I was young were really outspoken, and that was one of the reasons I adored them.

"I don't think my goal is to be liked by all," she continues. "And when you de-centre that as the primary motivation, I think everything becomes a lot more joyful."

As our time runs out, I throw out some random questions.

Is it true she's partially deaf? "Yes, I have 60% hearing loss in my left ear. If you sat on this side of me and tried to tell me a secret, I wouldn't be able to hear you."

What did she want to be as a child? "An obstetrician! I love babies, and I played obstetrician with my dolls, even if I was far too young to know how any of that works."

What would she call the movie of her life? "Oooh, I'd love to manifest an awesome title - like, 'Olivia Rodrigo lives the happiest, most joyful life any singer songwriter ever lived'."

Which role would tempt her back to acting? "Juliet in Romeo and Juliet. Obviously, the best love story of all time."

It's almost too appropriate – pop's heartbreak princess playing theatre's most tragic heroine. But, like Juliet, Rodrigo is mature for her age, taking control of her destiny in male-dominated spaces.

Before releasing her new album, she parted ways with her managers to assume control of her career, building a hand-picked creative team around her own decision-making.

It allows her to be nimble and decisive, skipping events like the Met Gala, which don't "inspire me or feel aligned with my values", she recently told the New York Times.

You don't have to be a genius to realise that integrity and authenticity are the anchors of Rodrigo's appeal.

Her songs have billions of streams because of their unflinching honesty. Rodrigo's as likely to reveal her ugly side – jealousy, obsession, self-destructive decision making – as she is to rant about her feckless exes.

"That's one of my favourite things," she says "I can write a song about being petty or jealous or super insecure, and I get it off my chest in a way that seems productive."

Maybe that's why she seems so healthy and well-adjusted, even in the furious glare of the spotlight. She even shrugs off London's torrential rain.

"It wouldn't be a proper English summer without it," she beams, hatching a plan to take her friends swimming in Hampstead's outdoor ponds.

She seems, in fact, pretty happy for a girl who's not currently in love.