The night before we speak, Sienna Miller came face-to-face with a 26.5-metre-tall version of herself at the Imax in Leicester Square, where Oasis: Don’t Look Back in Anger was premiering. The new documentary focuses less on brothers Noel and Liam Gallagher’s biblical feud than on the broad church that witnessed their rapprochement live. Among its subjects are a Brazilian student who named her dog after the band’s guitarist Bonehead, a South Korean freediver who says their lyrics helped her through depression, and Miller, filmed singing along to “Champagne Supernova” in the VIP section of Wembley Stadium – though she is quick to puncture whatever star power her cameo might have lent this patchwork portrait of fandom. “My fiancé [Oli Green], who is the shiniest-faced person you’ve ever seen, completely eclipses the fact that I’m in that shot,” she says with a laugh. “Everyone was like, ‘Oh my God, Oli’s in it!’ And I was like, ‘Yeah, I was there, too…’”

Miller is looking pretty radiant herself when we meet in a central London hotel, buffed, bronzed and butter-blonde in a fluffy white bathrobe. “Oh, no. I’ve been up breastfeeding since the crack of dawn,” she says in protest. “I just feel like a kind of lactating heifer.” Cue her latest co-stars: a herd of Highland cows. They appear in Barbour’s Shannon Murphy-directed autumn/winter 2026 campaign, in which old friends Lena (Miller) and Noah (Idris Elba) arrive at two different country pubs with the same name. What follows is a windswept comedy of errors: haphazard directions from the locals lead to a trek across the mountains as geriatric ramblers, screaming geese and a muddy sheep trailer draw them further into the wilds of the Scottish countryside. “My life has been a series of wrong turns that have led to something brilliant,” says Miller. “Life is a rollercoaster, and you just gotta ride it.” It helps, of course, to have good company along the way. “Idris and I have always had a laugh,” she continues. “But it’s always in passing, in that way London actors just cross-pollinate, so it was really nice to spend a bit of time with him.”

Miller’s relationship with Barbour goes back further still – to a childhood spent as a “country-bumpkin, horse-riding mud creature” in Berkshire. “I genuinely lived in a Barbour,” she says. “I proved it to the team because I still have the jacket I wore when I was little, with this very 1980s duck print inside. We thought we’d bring it back, but… who needs that?” The brand also features in one of her earliest – and most vivid – memories. “I got kicked in the face by a horse when I was five, on top of Martinsell Hill in Wiltshire. I remember my dad carrying me down the hill to hospital, bleeding into his Barbour, and the smell of the jacket,” she says – a scent that, even now, makes her feel protected. “Which I didn’t ruin, because it’s wax-coated.” (Though that is not Miller’s reason for comparing Barbour to a brand more conventionally used to mop up blood.) “In America, they say, ‘Have you got a Kleenex?’ No one asks for a tissue. And in England, when you go to someone’s country house, you don’t ask, ‘Have you got a wax jacket I could borrow?’ You say, ‘Have you got any Barbours?’ It’s part of our DNA. Baked beans and Barbours – and it’s never not been cool.”

“Cool” might be a word for which Miller has long served as a figurehead, but her own sense of it is now proving slippery. “From a young age, I loved clothes, and seeing what dressing up could do for my mother when she came down the stairs on her way out to dinner. Then my dad remarried, and his new wife had a daughter who was a similar age to me. She’d try to dress us in matching polka-dot leggings, tops and headbands… and I fucking hated it. I was like, ‘I don’t want to look the same as anyone else!’” There followed a period of rebellion – pink hair, mismatched lime-and-green trainers at secondary school – and early evidence that Miller’s taste would precede her. “I know I had a unique look, because there was a girl in sixth form who used to come and borrow my clothes when I was 12. Which was massive, though, admittedly, she was quite tiny.” Then came boho chic – you might have heard of it – which Miller basically invented from things she’d find at Portobello Market. Now, though? “I have completely lost my fashion sense. I’ve been pregnant or breastfeeding for nearly four years, and I think something happens to the brain – or you’re so fucking tired – that you sacrifice your identity. You’re just ‘mother’... and not in the Gen-Z way, as much as I would like to be.”

This is, of course, nonsense. See: the neon Conner Ives housecoat she wore to the Serpentine Summer Party in June, which set this website’s traffic dashboard alight, or the jacquard Colleen Allen dress she wore for an appearance on the month before. “I know I have a good aesthetic,” she adds. “I can tell when something’s lovely and when it’s not, but when it comes to putting clothes together myself, I don’t know – I’m just in a strange place. I literally have to text my friends who have great style and ask them to help me.” Most recently, she turned to Alexa Chung, who is engaged to Tom Sturridge, Miller’s former partner and the father of her eldest daughter, Marlowe. “I texted her and said, ‘I’m going to the Oasis premiere tonight. What do I wear?’ And she said, ‘If I were blonde, I’d be in a slutty black dress all day long. Find something Carrie Bradshaw and put it with a mule.’ I replied, ‘Googling mule…’” Surely Miller knew what a mule was? “I’m just really questioning all this, because I’m just so underslept and I kind of don’t know who I am. I also have massive boobs, which I don’t know how to dress. Anyway, this is far too much information…” Duly noted – so back to Marlowe, who, having just turned 14, now has access to both Sienna Miller’s Alexa Chung’s closets?

“She’s got it all going on. She’s very cool,” Miller says. “She says she likes bottom halves from me and top halves from Alexa – and she doesn’t hold back with the criticism. The other day, I put on this Chloé suede jacket – a really beautiful blue, gorgeous, cape-y thing – and she just went, ‘Napoleoooon!’ So I took it off. I do feel like I’ve got this great usurper in my midst…” Speaking of Chloé, I wonder if Miller has heard the rumours that the brand might be looking for a new creative director, with Chemena Kamali tipped to move to Saint Laurent. Her mouth forms a perfect O. “They must have paid her a fuck-tonne…” Whether that amounts to anything more than internet tittle-tattle remains to be seen, but before Miller trades in her suede capes for Le Smoking jackets, here’s how she is actually dressing for the season ahead: “Jeans and a nice jumper and a Phoebe [Philo] shoe,” she says. “But, really, what I’m most excited about is people breaking away from the social-media uniform. I’m yearning for that individuality – for people who don’t have a playbook and have to go and cobble things together. I’m looking forward to trying to find that myself.” Otherwise? “I like a pint of Guinness and I love a pork pie; the Devonshire in Piccadilly, the Elephant and Castle in Notting Hill – and come on Knebworth!”