When you want a measure of a child actor's potential, look at how convincingly they cry in character. Leonardo DiCaprio was great at it – and he's doing all right for himself. Do a YouTube search for Tamsin Egerton and you'll find an old clip from a supremely awful-looking TV mini-series called The Mists of Avalon. There's Anjelica Huston, dressed up as a high-class medieval witch. And there's young Tamsin, just 11 years old, saying goodbye to her baby brother, King Arthur, played by little Freddie Highmore, the sickly-seeming boy that your mum fell for in Finding Neverland. The pair must part for the sort of spurious "magical" reason that often provides plot points in B-movie fantasies, but Tamsin's having none of it. She pleads, she protests, and her eyes well up with tears, which finally tumble down over her still-chubby 11-year-old cheeks. Even with the distinctly dodgy voiceover and the kitsch-as-hell strings keening away in the background, you think: "She's not half bad at that."
It's a bit disconcerting to watch this clip, and then be introduced, a couple of hours later, to the real-life, present-day, 5ft 10in Tamsin Egerton, all anatomically improbable 36-inch legs and bombshell-blonde hair swishing about the place as if to punctuate her frequent exclamations. It's lucky the private members club; we're meeting in seems free of precarious Ming vases; in her Labrador-like enthusiasm, one of those long, shapely arms could do some expensive damage.
Last year Egerton came 10th in the Independent Critics' list of the 100 most beautiful women in the world. At 19, she's only a year older than Emma Watson, who plays Hermione in Harry Potter, and who still incites heated debate over whether it's decent to fancy her yet. And yes, Egerton is beautiful. And yes, she could do the crying thing from an early age. But is that it?
In case you're still wondering who she is, Egerton (pronounced "Edge-erton") found fame as Chelsea, the posh, dim sex-pot in last year's exuberant remake of St Trinian's. The film heralded the arrival of a gaggle of young British actresses with looks and ability to spare, all of whom may soon be auditioning for the same roles. Egerton says she aspires to the variety and longevity of a Judi Dench ("everyone's going to say that though, aren't they?") or a Julie Walters. But carving out a career with that sort of competition is going to take some talent – and possibly a ruthless streak.
Of the St Trinian's alumni, Egerton's not the one who's now a Bond girl (that's Gemma Arterton), although she's fond of martial arts and old-fashioned gents, so it could well be on her wish-list. She's not the one who was in Venus with Peter O'Toole (that's Jodie Whittaker), although she would go topless like Jodie if the part demanded it; she's done it already, aged 16, in Keeping Mum. And she's not the one who was a Bennet sister in Pride and Prejudice with Keira Knightley – that's her best gal pal and flatmate, Talulah Riley, with whom she's been doing a pretty half-hearted job of competing.
"Talulah and I audition for the same parts all the time, but it's not a problem," she gaily insists. "People always think 'Ooh, competitive actresses', but Talulah and I would rather each other got the parts. In every audition, I'm like, 'Talulah's great! She'd be perfect for this!' And she does the same for me. I'm not a competitive person. I'm trying to cultivate a long-term career rather than get every job right this minute. That'd be putting too much pressure on myself. I'd go crazy if I thought like that."
What Egerton does, so far, is a nice line in teen nymphos like Chelsea (who runs a sex chatline from her dormitory) and Holly, her character in Keeping Mum. Her next project – the lead in a new Jilly Cooper adaptation – won't exactly buck the trend. Octavia is the story of a self-absorbed seductress who sets out to steal her best friend's fiancé during the long, hot summer of 1976.
"I have to be careful not to do so many comedy airhead roles from now on," she agrees. "I've turned down a couple already. Chelsea was a bit of an airhead, and Octavia is as well, but it's a coming-of-age story, so at the end she realises the error of her ways. I took it because it's the title role, an older, more sophisticated part than Chelsea, in a serious piece with a hint of comedy. I want to explore more serious roles." Yeah – everyone says that, too.
"My looks could be a problem. Acting is an aesthetic career, which is annoying. Normally I'm a brunette, and I always used to do classical roles, but as soon as I turned blonde for St Trinian's, people started to see me in a different light. I can do comedy, so people are going 'Ah! Comedy slut.' And I'm like, 'No! Look outside the box! You could dye my hair dark, or cut my hair!'"
Egerton, originally Tamsin Egerton-Dick, started acting aged six, when her older sister Sophia joined a local youth theatre. "The whole reason I'm acting now is because I wanted to be doing what my big sister was doing," she says. "If she'd wanted to be an astronaut, I'd probably have done that." Nowadays, big sis is a scuba instructor on an island in the South Pacific. Tamsin, thankfully, didn't follow her there. Instead, after a few small acting jobs, she landed her part in The Mists of Avalon, and dropped the "-Dick" from her surname to avoid confusing American audiences. A year later, she was on the West End stage as Mary in the RSC's musical production of The Secret Garden. The grown-ups who worked with her at the time say they could already spot her movie-star potential.
At 16, she had her first film role in Keeping Mum, a Brit-com with Rowan Atkinson, Kristin Scott Thomas and Maggie Smith. Her brief topless scene caused a stir, but her parents didn't seem to mind. "It's not like I was doing a topless shot for a lads' mag. I'm not prudish; if a part calls for it, it's only some boobies." This game approach must have served her well when it came to carrying out the unruly antics required of the girls of St Trinian's. The fun of that establishment, though, was a far cry from her real school experiences.
At Ditcham Park, a private day school in her home county of Hampshire, Egerton was the sort of girl whose illicit drinks cabinet contained tea bags, not vodka. "I'm addicted to tea," she explains. "I drink, but not to get drunk; I don't smoke, and I don't do drugs. I'm a bit of a good girl. But I get a headache if I don't have a cup of tea. I'd smuggle in a kettle, tea and powdered milk so that my friends and I could have cups of tea at lunchtime.
"It was a country school on a hill in the middle of nowhere. I had an amazing time there until I started to take time off to work. The kids obviously thought I was just jet-setting off to exotic locations. But actually, I was 11 years old and doing a nine-to-five job with adult pressures. I don't think the other kids understood, so I was bullied. My teeth got knocked out once; I was blindfolded and someone smashed me into a pole."
The pupils were half right, however, about those exotic locations. Egerton travelled the world with her mother, Nicky, who was also her chaperone. "I gained so much more than I lost by being a child actor. We went to South Africa, St Helena, Ascension Island, Montreal, all around Europe. We've seen the most incredible places. I'd be mortified to think my parents might have turned all that down to have me go to school all day every day, in a little box where everybody does the same thing the whole time and you get told off for speaking a bit louder than normal. I don't like that life at all."
After spending so much time in the company of adults, St Trinian's was, she admits, the first time she found herself with a group of girlfriends her own age. She's particularly excited about her recent move, into a mews in Marylebone, with Riley, who's 22. "I've lived in London for two years," she says, "but this is the first time I've lived with a friend ... At the moment it's just perfect; we're in our honeymoon period. We have friends round, we entertain." Cor, that's a bit grown-up, isn't it? "I'm a bit of a middle-aged woman, actually. I'd rather have a cream tea and go shopping in Waitrose than go to a bar."
This sounds a bit anomalous from a girl who, along with her housemate, is fast becoming a gossip-page favourite. She appears to be blissfully unaware of all her press appearances, all those Grazia "get the look" features and red-carpet paparazzi shots. But surely she remembers all the shindigs she's been to recently? As I leave her, she's clutching a posh-looking party invitation. Just leaf through the glossies and there she is, tearing it up with Talulah at the RA's Summer Exhibition opening, joshing with Jenson Button at the Cartier International Polo Day, at the launch of Pharrell Williams' clothing line in Harvey Nicks, at the Tatler Summer Party, at an Aids charity ball, at the premiere of The Dark Knight. And that's just the past month. Not bad for a girl who prefers to stay in.
Despite the allegedly amorous attentions of F1 racing drivers, Egerton is currently single. Earlier this year, she split up with her long-term boyfriend, Andy Jones, a semi-professional footballer and former Hollyoaks cast member, whom she met on the set of Keeping Mum. "It was wonderful while we were together, and we're still very amicable," she says. "I saw him yesterday. We went and bought shin pads for him [Jones is about to start another season up front for Enfield Town]. Apparently with shin pads it's the smaller the better. He was explaining all about small shin pads. I'm quite glad not to be going to his matches any more, because I only really did it to be supportive. I'll happily sit down and watch a football match but I won't watch Soccer AM, Soccer PM, and Sky Sports News five times a day. You can tell this has really affected me, can't you? Sky Sports News on ALL THE TIME?! That's something to cope with. But it's very sweet when someone's got a hobby."
In fact, Egerton's more of a rugby fan, like her dad. She's more Jonny Wilkinson than David Beckham ("He's a real man"), more Colin Firth than Russell Brand ("He's an old-school gentleman"), more Ascot than Glastonbury. "Although I don't mind mud, because I'm a country bumpkin. Walks, rambling, bed and breakfasts, dogs, Land Rovers – that's me. Staying up all night and passing out drunk in a tent really isn't."
Her one non-acting job was a brief spell as a babysitter for three young children. "That was difficult. I'm neurotic about children. I see dangers everywhere – sharp corners, stairs. They put me in charge of the most precious things in their world, and I was only a teenager. I was thinking, 'I can't even look after myself! What if they choked on a button when I wasn't looking?!'"
Considering how easily she's distracted, such a scenario isn't beyond the bounds of possibility. But for the near future at least, Egerton's chances of getting embroiled in childcare are slim. She's clearly a consummate professional, one with proven talents that St Trinian's, to be perfectly honest, barely tested. And, gosh darn it, she does seem thoroughly lovely. But where's that ruthless streak? How will she out-perform her peers? Maybe she ought to be a bit nastier, a bit more hard-nosed. Then again, maybe she is. Maybe she's secretly a big old meanie, but just a damn fine actor with it. Either way, I'm willing to be convinced.
'Octavia' is broadcast later in the year on ITV1