“Look, look, look,” she says. “Oh my God!” From a large-screen TV, Stone’s GAP commercial is airing, the one where she’s hawking white jeans by crooning her version of “The Right Time” for a few cute friends during what looks like an impromptu backyard concert.

“That is so crazy,” she says, as images of swaying bodies flash across the screen, everyone decked out in summer Gap wear. The camera pans to her pals, then to her face, then to what looks like her swaying rear.

“That’s not my bum!” Stone screams.

“All these bum shots? They’re not mine. They’re like other girls. That’s not my bum, I promise,” she says, watching the screen with morbid fascination. “They totally make it like it is.”

This little editing trick genuinely freaks Stone out. Her face betrays the insecurity: Why would they do that? Why take the voice and not the rump? And is there something wrong with her rear end?

“I get really nervous. Apparently, I need a J-Lo bum or something,” she says. Then to the publicists waiting outside she screams: “Bring in the implants, girls.”

The lesson: You might be young, willowy and talented, but show business can still rip you down. Chalk it up to one more thing the 18-year-old has had to learn during her wild ride from southern England.

Since winning a BBC talent contest at age 14, she has released two hit albums, snagged two Grammy nominations and serenaded President Bush. She calls Elton John and Tom Cruise friends, been dubbed a diva by VH1 and gets tips on vocal spray from Sting.

But, like the GAP ad, she concentrates on the flaws.

“When you step back and you look at it, it’s like, ‘Wow, that’s really cool that I’ve achieved this much.’ But I haven’t really achieved a lot, a lot, a lot compared to a lot of other singers,” she says.

“It does make me happy when I look at it. But, still, I’m very critical of myself in every situation, even if it’s something really good. I always find one note wrong. Always. Every single time.”

Fans apparently haven’t heard them. Stone’s first CD The Soul Sessions – a 10 song collection of little-known R&B songs from the ’60s and ’70s – zoomed up the charts in 2003, fueled by her reworking of The White Stripes‘ “Fell in Love With a Girl.”

The album also brought disbelief in some corners – that a white chick from rural England could sing like Aretha Franklin. That a mere teen could wrap her voice around the pain of soul.

“It’s weird when people say to me, ‘How do you sing that – you’re only 18?’ I’m like, ‘Well, how many times did you fall in love by the time you were 18? Like 50 million times?’ And, you know what? The first time that you fall in love is the best and the worst. And the first time you break up it feels like, Oh my God, you will die tomorrow.”

Still not sold? Then you’ll have to deal with Patti LaBelle.”Everybody goes through the same thing – it doesn’t matter what color you are. And Joss Stone just happened to wake up one morning with all this soul that had to come out. So people better not be thinking it’s a gimmick. It’s for real,” says LaBelle, who has worked with and mentored Stone.

“She was born like that. She didn’t practice it. It just happened that she opened her mouth and a big black woman comes out,” she says. “The girl can just sing.”But Stone herself seems somewhat shaky on this. Take the last Grammy Awards, where her duet with Melissa Etheridge on a Janis Joplin medley of “Cry Baby/Piece of My Heart” was an instant crowd pleaser.

“I saw it and I was like, ‘What the hell are people talking about?’ At the beginning, I hit the worst note. Any singer would tell you. The first cry was so way out of it and then it was cool, but I always manage to do that to myself. I beat myself up for no reason,” Stone says.As is customary for Stone, today she is barefoot, wearing a long gypsy skirt and tank top, her wrists covered in bangles and a tiny stud gleaming in her right nostril. The look – helped by her long, wavy blonde hair – is pure hippie chic.

“I don’t like wearing shoes. I prefer going barefoot. I feel like I’m at home, you know? I don’t go home very much so I try to make myself comfortable in every way I can,” she says.Stone’s current CD, “Mind, Body & Soul” – with 11 of the 14 songs co-written by Stone – combines a classic soul sound with contemporary pop beats, sort of like soul for the iPod generation.Soul, she says, was her main musical diet while growing up, the thing that moved her adolescent heart in her bedroom in Devon. She has tried to sing in different styles, but it never worked.

“It doesn’t matter what song I’m singing – I could be singing rock ‘n’ roll or reggae – it’s just the way it comes out,” she says. “I get into it and I feel like it sounds bad to me when I don’t.”Ask her about her own voice, though, and Stone demurs. “I really don’t feel it can do that much, to be honest. Some people like the sound of it. I don’t know,” she says uncomfortably.”Really, I can’t do (expletive) with my voice. I can’t go really high. I can’t do runs after runs after runs like Beyonce. I do, like, one run. I just sing – I don’t do all that stuff.

“I think anybody who puts their emotion – their whole heart – into it, you will feel it. It’s like if the worst singer in the world comes and sings a song and they’re almost crying, you will cry. You will feel it. It doesn’t matter whether you’re a good singer or not. That’s what I do.”

The singer is touring Europe through August, then has dates in the States in September and October. A tour of England starts in December. What else is on the schedule for Stone? Probably an album. She admits to being a little sick of continuing performing songs from the last one.”I have so many ideas. Maybe I’ll go a little funky, maybe I won’t. Maybe I’ll record it like this, maybe I won’t. I’m really not sure. It’ll be a nice surprise,” she says.