Rosamund Pike and Robie Uniacke in the front row Christian Dior show, Autumn Winter 2016, Paris Fashion Week. Credit:Swan Gallet/WWD/REX/Shutterstock "It's really about the triumph of love," says Pike, who plays Ruth opposite David Oyelowo's Seretse. "I've always had this instinct that if you're loved, and you love, it can give you tremendous strength and bring out qualities in you that you didn't know you had … [Ruth and Seretse] were up against adversity that most people would cave in at, and they didn't. "Falling in love is always a risk on some level, and Ruth and Seretse took a risk to a whole other degree in the time they were living. What's interesting about this film is that you're seeing animosity and suspicion and hostility on both sides." Pike has some understanding of what it's like to be in a relationship viewed with disapproval by others. Uniacke is 18 years older than her, twice divorced, and has four children from his previous marriages. "My partner is older than me, so, you know, not everyone goes for that – and it feels wonderful to stand true and own it, and say, 'This is right.' She hopes A United Kingdom will give other couples the courage to also stand their ground. "There are people worried for you because it's not the norm … [but] love is never about ticking the boxes that you think [are right]," she explains.

"When you find the person who suits you and complements you and is your partner in the adventure of life, it's often never who you'd expect, but it's the combination that works." For Pike, Uniacke's wit, intelligence and irreverence – not to mention their great chemistry ("That's always been the case and that continues") – makes him the one for her. "I want someone who pushes me to explore and try new things, and I think we do that for each other." All tousled blonde hair, wide blue eyes and peaches-and-cream complexion, Pike seems to personify the posh, remote English girl with her poise, clipped British tones and Oxford education. Yet it's clear she's a lot more fun – and far more human – than that stereotype. For Halloween, for instance, she wrapped older son Solo in toilet paper to be a mummy and took him trick-or-treating, while two-year-old Atom stayed at home with his dad. "We let the baby open the door [to the trick-or-treaters] and everyone was like, 'Sweet, it's just a little tiny boy" and then my husband would jump out dressed like a gorilla," she recalls with a peal of laughter. "We have a great time with our boys, we have real adventures, we have fun together as a family." An annual trip to China is their newest source of adventure, and they spent last Christmas in a camper-van out in the desert at Joshua Tree in southern California. This Christmas, Pike is filming Entebbe, about the 1976 hijacking of an Air France jet. (Preparing to play one of the German hijackers, Pike says she's so into the movie already that she's wearing flares and a Lurex top as we speak: "I've become a '70s stereotype!")

A family ski trip is also planned. Pike is disarmingly daunted by the prospect of kitting up for her alpine debut. "If I buy something nice, I'm going to look like a beginner wearing something fancy," she confesses. "I'm going to look like an idiot, and I just don't know how to do it, so it's always put me off." An only child, Pike spent the first seven years of her life moving around Europe as her parents' musical careers dictated before being sent to boarding school in Bristol at the age of 11. She enjoyed a lot of one-on-one creative time with her parents, which is why she thinks she'll stop at two children of her own. "I want to give that to my kids and I feel that with two, I can give them both that. With more, I maybe couldn't." Having children has taken her career to a new level, she says: being a mother has given her greater physical courage and made her acting more spontaneous. "Your emotions just sit closer to the surface – maybe because you're more tired, I don't know," she laughs. "I feel I can access my rawest self much more easily because there's no time to put up a mask any more, there's no time to think if you're a working mother. "The other thing I can't do: I can't ever over-analyse things at the end of a day's shooting. I walk into the house and I let it all go, because I have to get stuck back into whatever's going on."

However, she admits she struggles sometimes to transition out of roles in which she's been immersed for months. "I came back from doing this film in America [the western Hostiles, with Christian Bale] and I had to go straight into doing press [for A United Kingdom] and I was a mess because I thought, 'I don't know how to be myself any more'. I think acting is quite a dysfunctional thing – you're sorter happier being someone else." It was following the birth of Solo in 2012 that Pike's acting career soared. Although she'd been working solidly since university, making her film debut at 21 as a Bond girl in Die Another Day, and going on to play Jane Bennett in the 2005 movie version of Pride & Prejudice and opposite Tom Cruise in Jack Reacher, she was still relatively unknown before starring in Gone Girl. This was precisely the reason director David Fincher cast her as Gone Girl's woman in question, Amy Dunne, whose husband becomes the prime suspect when she disappears. The movie delivered Pike film-star fame she never envisaged when she was at university in Oxford. Rejected by every drama school she applied to, she ended up graduating with a degree in English literature but was still doggedly determined to be an actor. "I never thought I'd have a career in film – I knew nothing about film," says Pike, who grew up without a TV at home. "I knew theatre, and I'd thought that's where my life would be." She also never thought she'd be Hollywood-famous. "I think I can say I'm quite famous now," she says matterof-factly. "It sounds an odd thing to say, but I am. I realised I was when I was in the grocery store with my children in America and this woman came up to me and said, 'I just had to say you're one of my favourite actresses.'

"I must have looked shocked and she said, 'We do recognise you, you know.' She picked up exactly on what I was thinking, which was, 'Gosh I didn't think anyone knew who I was'." Pike's family accompanies her on every shoot, and she thinks her elder son is beginning to understand what his mother does for a living. "I always think they'll appreciate growing up seeing a parent pursuing her dream. I think they have a very rich life because of it." From next year, Pike's life will become a bit more settled, as once Solo starts school she'll seek projects based around London. "That's the beauty of it; this year I'm working really hard but next year I'll take a good few months off and make sure I'm there to pick him up from school and drop him off and just be there... There will be times when career comes first and times when you can just say, 'Right, I'm going to put the children first'." A United Kingdom opens on Boxing Day