She's dark, sarcastic, and bashes life in Nowheresville ... so how did Kacey Musgraves become country music's hottest new star?

The girl from Nowheresville, Texas, has the crowd in a tizzy. "London! You are mental! In the very best way," twangs tiny Kacey Musgraves from centre stage. With two Grammys for her first album under her belt, she is country music's hottest new star – and she's milking every minute.

Her British fans have doubled in number since she was last here, nine months ago; the standing-only stalls look twice as packed. This warm July night is her last UK date for a few weeks, before she's back to play BBC Radio 2 Live in Hyde Park festival on 14 September, alongside Debbie Harry and Chrissie Hynde on a mixed bill that suits Musgraves perfectly: she might be "too country" for some US radio stations, but she doesn't do musical genres – something she thinks she and the British have in common.

That and a shared sense of humour. We're "super-witty", she says, which is why we get the "sarcasm" in her songs. So maybe the joke's on me when, instead of relaxing off stage, beer in hand, for a chat, I'm bumped to the front of a post-gig selfie queue. There's nothing like gushing "loved the show" as a tiny arm wraps around your waist for trashing your journalistic credentials.

So, I'll admit: I'm a fan, along with a string of Nashville greats from Willie Nelson and Ryan Adams to Alison Krauss and, er, Katy Perry. For it is the latter, that multi-coloured pop princess, she of the 55 million Twitter followers, who was responsible for lighting the firework under Musgraves's swift rise to stardom. Or should I say Taylor Swiftian rise to stardom? Same Trailer, Different Park, Musgraves' first album for Universal's Mercury Records, scored a surprise victory over Swift, country's biggest crossover queen, for Best Country Album at the Grammys, proving that this newcomer is very much, as Radio 2's Bob Harris puts it, "the real deal".

Awkwardness aside, the "meet and greet", or M&G as it's known, with the 25-year-old's acolytes is illuminating, summing up both country's new face and the new face of UK country audiences. The eclectic few have been picked at random from emails sent in to her website and span the old guard – an LP- cradling, middle-aged "stalker" (his wife's words) – to the new: teenage Musgraves lookalikes in denim cut-offs and cowboys boots.

A bit like my balcony-seat neighbours back at the concert, in fact: the Radio 2 presenter Jeremy Vine and his teenage daughter, united in their adoration. Like all the country greats, Musgraves has a knack of writing songs that speak to everyone, and she cues a new one, "The Trailer Song", about life lived up close to your neighbours, by asking the west London crowd, "Where the fuck are the trailers? I've seen like, one, since I've been here. Which is weird because I'm from Texas. But I know you all have noisy neighbours." And the crowd cheers.

Another new song, "Cup of Tea" – "appropriate for this side of the world, huh?" – celebrates everyone's right to be different: "You can't be everybody's cup of tea." But country's right-wing core doesn't hold with people being too different: Musgraves caused a Southern storm with "Follow Your Arrow", which offers the controversial message that you should do what you want and love whoever you choose: "Make lots of noise/Kiss lots of boys/Kiss lots of girls/If that's something you're into…/Light up a joint/Or don't/Just follow your arrow wherever it points."

Musgraves proves they don't call her Spacey Kacey for nothing when, five days later in a London hotel, she keeps me waiting for our interview for more than an hour. I guess a 2.30pm start was optimistic when you're fresh off an overnight tour bus from Dublin. She walks in carrying a plate of sliced fruit and veg, which, true to pint-sized star tradition, she barely touches. Again, it's the wrong backdrop for an interview: had I known we were meeting for "breakfast", I'd have suggested a greasy spoon. The fatigue – the ferry over was in the early hours – leaves her groping for the odd word, but her thorough replies are impressively professional. Or perhaps it's just that legendary Southern politeness shining through.

It's almost two years to the day since Katy Perry tweeted her out of the blue; Musgraves was crossing the road in Dublin at the time. Perry had heard her song "Merry Go Round", which disparages life in smalltown USA, "and loved it, so she shared it with her fans. It made my world explode for a little bit."

Then, last year, Perry asked Musgraves, who, like most Nashville hopefuls, started out penning tunes for the great and good, to write for her new record. The upshot was Musgraves spending this month opening for Perry on her Prismatic World Tour in the US and Canada, their second shared gig this year. The pair also hooked up for a US TV show that pairs pop stars with country singers – CMT Crossroads – and appeared together on the cover of US music magazine Billboard. Others who have appeared on Crossroads include Elton John and Ryan Adams, Def Leppard and Taylor Swift, and Lionel Richie and Kenny Rogers.

Musgraves also, indirectly, has Perry to thank for "Follow Your Arrow". She offered Perry the song, which stemmed from a few lines she'd written for a friend who was moving to Paris: "I'd given her this little arrow necklace and just wrote like a dumb little poem on the card and I had come up with a little chorus for it." But Perry thought Musgraves should keep it for herself. "She was like, 'That's great, but it seems like something that you would totally say.'"

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More recently, there has been talk of writing with Ryan Adams in the autumn after he, too, tweeted her. And she's hoping some ideas she "threw around" with Willie Nelson while touring with him earlier this year pan out into a song.

It's all sounding pure Nashville – that's Nashville the hit ABC show, which aired here on Channel 4 (featuring a Musgraves number, "Undermine", in an early episode). Especially as the singer is about to use the "mailbox money" she made from co-writing Miranda Lambert's hit "Mama's Broken Heart" to buy her own house. Musgraves even has a gay co-writing friend in fellow country singer Brandy Clark, ticking off two of the drama's second-season plot lines. And yes, it's as big a deal not being straight in country and western's heartland as the Nashville scriptwriters portrayed, which explains why Clark lacks the accolades and airtime she deserves for her own critically acclaimed work.

Musgraves blames the lack of diversity in country on being "stuck in this really constrained subject matter. It seems like there are five things people sing about: partying, drinking, driving down a dirt road…" The irony is that "in the country genre you can be, quote unquote, 'too country'". Meaning? "It's the instrumentation. If you don't sound like you're trying to be some kind of rock spin-off, you're too country."

She's pushing those boundaries but thinks there's "unfortunately" a limit to the kind of subjects you can sing about and still be mainstream. "But hopefully that goes away because I think the best music is inspired by every emotion possible, not just a party or a truck."

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It was moving away from her own small village of Golden, a Texan farming community about 80 miles east of Dallas ("just a church, a little market, a bunch of stray dogs, and some sweet potatoes") that made Musgraves realise it's OK not to be everyone's "cup of tea", as k she puts it. "Things like, y'know, one of my best high-school friends coming out to me at a young age, and me realising that he had gone his whole life not being able to be who he was made me want to be like, I don't care if people typically sing about this or not. We're all driven by the same emotions no matter what we're into, so that's the kind of music I want to make."

American music writers talk of a female-led backlash to Nashville's dominant "bro-country" music scene and the beer-swilling, broad- chasing likes of Jason Aldeans and Luke Bryans. But Musgraves insists it's less about girls versus boys and more about "being relevant, and inspiring, and good". She nominates Eric Church as a fellow country rebel who's forged his own path. "He doesn't sing about the typical things that most people do [so] radio didn't want to play him at first. But he built a really strong fan base, and now radio has no choice."

Perhaps, with all the talk of crossover stars, genres aren't still relevant? "Yes and no. I think it's important to know who you are and what you're trying to say, but I don't think that should limit you. A lot of times in America, music is limited by the radio genre, which isn't really a genre, but people want to be played on the radio and they tailor their music to that, unfortunately." She loves that we blend our genres here, not least at festivals such as Glastonbury, which Musgraves played this year, the day before her heroine Dolly Parton. Like Dolly, she's a "three chords and the truth" gal. Giggling, she adds, "I have a two-chord song."

With two Grammys for that first album (out of four nominations, tying her with Lorde and Taylor Swift for the most nominations for a female artist), Musgraves feels there's much to live up to with her second. But at least she won't have to recast her mould: unlike the Nashville drama, where Hayden Panettiere's character Juliette Barnes is pitched in a battle with herself and the music industry to ditch her manufactured, teen-friendly early image, Musgraves made artistic freedom a condition of signing to Mercury. Working with two first-time producers, "We got to create something really organic from the ground up and prove ourselves that way. I'm thankful, because you can't backtrack from that now. I would laugh in their face if they tried to tell me to do something different now just for more numbers."

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This differs markedly from the usual formula: "Playing it safe to try to get a wide appeal first to get as many fans as possible, and then get more edgy: I totally disagree with that; I think it should be the opposite." Hence the Nowheresville-bashing "Merry Go Round" as her first single, "even though [the label] were like, 'This isn't what a debut artist would typically say. It's downbeat, depressing, and dark. And we need up-tempo. Something for the summer.' And I was like, 'No. Don't you want me to stand out?'"

On stage, and in the flesh, she hits the alt-country note visually as well as orally. "My style is a whimsical take on traditional Western wear." Her band have suits in the "Nudie style" – as in Nudie Cohn, the Ukraine-born American tailor who first mixed Nashville and Hollywood – or Porter Wagoner, the embroidery-and-rhinestone-bedecked singer. "But we added twinkly lights to bring them into a sort of futuristic world," she says. Today she wears high-waisted flares, a slim-cut floral shirt, and the same cream patent platforms she wore to open her Shepherd's Bush show, before casting them aside in favour of bare feet.

After a packed fortnight that took in Norway, Italy and the Netherlands as well as the UK and Ireland, she is yearning for home, and happy to be heading back the next morning, not least to see her beloved dog Pearl. She whips out her phone to show me a picture. "She's got three legs; she's a rescue."

In a neat twist that suggests she's not nearly so down on the stigmas of small-town life as "Merry Go Round" would have you believe (chorus: "We get bored, so, we get married/ Just like dust we settle in this town"), that house Musgraves is buying is her old family home in Golden. "It's very basic, but very cute, just two bed, one bath. I'm really excited just to be able to go back home and sit where my sister and I shared a bedroom for years; it's kind of crazy." Her sister, Kelly, is even getting married in the front yard in October to complete the image.

I guess the get-out is that there's little danger of either sister settling for settling in that town. Especially not now the UK is a firm fixture on Kacey's tour schedule.

Kacey Musgraves is at BBC Radio 2 Live in Hyde Park, London W2, on 14 September