Conrad Khan was filming in a fried chicken shop not far from where he actually lives. In the scene, his character’s bullying schoolmates come in and demand he hand over his chips. Then, as shooting began, Khan’s real-life schoolmates turned up, curious about a film crew in their local hangout. “They looked in,” laughs Khan, “and did a double take. They were so confused. They didn’t know I’m an actor. So I was quite proud. Yeah, quite cool.”

County Lines, the film Khan stars in, portrays another kind of double life. Khan plays Tyler, a quiet, troubled 14-year-old with an absent father, a hard-up mother, and little to look forward to. He is rescued from the chicken-shop bullies by local hood Simon, played by Harris Dickinson, who has a flash car. Simon buys Tyler trainers and offers him the chance to earn some money. As the title suggests, he is soon skipping school and hopping on trains out of London, ferrying drugs to regional dealers.

Before it was overshadowed by Covid, county lines was an urgent national issue, widely discussed in the news, chronicled in documentaries, but also semi-glamorised by such drill rappers as Frosty, aka Theo Beckford, whose 2019 tracks County Lines 1 & 2 proved to be borne of experience. Frosty was imprisoned for drugs offences.

Nobody could accuse the movie of glamorising its subject. County Lines is visually lyrical but unflinching and often brutal, in the social-realist tradition of Ken Loach and Alan Clarke. And Khan puts in a head-turning performance: a study of boyish vulnerability, loneliness and frustration. “That’s what these drug dealers look for,” he says. “Kids with no emotional support. Kids who don’t really have anyone to talk to, who almost feel like they have something to prove and they haven’t been able to. That’s who are easiest to prey upon.”

County Lines is also born of experience. Not Khan’s but that of writer-director Henry Blake, who worked in a pupil referral unit for a decade. In the past five years, Blake has seen the human toll of county lines first-hand: the deprivation, the exploitation, the violence, the missed opportunities. As such, he had some grim stories to prepare Khan for the role. “Socially I can’t say there are any parallels between Tyler and myself,” says Khan, who was actually 18 when he made the film. “But emotionally, yes. That desire to feel accomplishment, to provide for the family – that’s stuff that any teenage boy has felt. I think that’s what makes the film so relatable.”

At 14 he was climbing trees, he says, and getting his first taste of serious acting. He had a small role in the 2016 fantasy epic The Huntsman: Winter’s War, as the young Chris Hemsworth. “That’s what made me fall in love with the industry.” He does not have a showbiz background. His father works in health, his mother is a lecturer in fashion. His grandfather is from Pakistan, hence the surname.

It was a chance suggestion from a barber in Muswell Hill, who’d cut hair for film actors, that turned him towards the profession. He got the lead in his school play, Charlie and the Chocolate Factory, spent a few years with the Arcola Youth Theatre, and won parts in commercials, short films and TV, including an episode of Black Mirror.

All the while, Khan led his double life. “I used to tell my agents to get me auditions any time after 3pm so I could finish school first.” He made County Lines while in his final year of A-levels. The month-long shoot was especially tough. “It was four days on, one day off. Then the day off I was in school.” Added to which, filming often involved grim situations. Lying in a cold Essex marsh in the middle of winter, for example.

“I was head to toe drenched in mud and it was seeping inside my clothes and it was freezing cold. It was December, at night, lying there, doing the scene over and over. So, a little bit method.” He’d gladly do it again, though. “The reason I love it is to explore my emotional capacity, to find roles that push me as far as I can go. I think that’s what this film did. To my absolute limits. And I loved it.”

The career/education balance is still an issue, with Covid an extra complication. County Lines was scheduled for release this April. Khan then had acting jobs lined up for the rest of his gap year before starting university. “The timing worked perfectly,” he says. “Then, like everything else in the world, it all just came tumbling down.”

The acting has at least restarted, albeit temporarily. He’s been shooting BBC’s Baptiste for the past two weeks. “It’s very strange being on set in Covid times,” he says. “I still don’t know what anyone I’m working with looks like. And there’s an entire new profession which didn’t exist six months ago, called Covid coordinator. Their job it is to tell everyone to keep away from each other and stop kissing and holding hands and taking masks off.”

Khan also started university a few weeks ago, albeit remotely. He’s doing film studies. “It’s all online, which kind of suits me ’cos I’m still working, so it allows me to do both at the same time. I’ve had to pause a lecture on François Truffaut to talk to you.”

Sooner or later, it seems likely Khan will be presented with a dilemma. Supposing a great role came up halfway through his studies? “This is what me and my agent and my parents have been talking about non-stop. So yeah, I’m glad you brought it up. I don’t know what I would do. I’m taking a risk going to uni, but I just feel like I want to be in an academic environment and around like-minded, similar-aged people again, because I do miss that.” He pauses, then adds: “But I’d probably take the role.”

• County Lines is released in cinemas and digitally on BFI Player and Curzon Home Cinema on 4 December.