As the UK once again awaits a new Prime Minister, attention is fixed on the latest Westminster upheaval, but few political shocks in recent British history match Brexit.

A decade ago this week, the UK voted to leave the European Union. Veteran documentary filmmaker Norma Percy’s Brexit: A Very British Civil War relives events with unprecedented access to the movers and shakers from both sides, the Leave camp and their opponents who backed Remain.

“Our brief was to find out how Leave did it,” Percy tells Deadline of the BBC two-parter. Interviewees include former Prime Minister, David Cameron, whose surprise 2015 election victory forced him to make good on his pledge to hold a referendum on EU membership.

Boris Johnson, who was then Mayor of London, George Osborne, Chancellor of the Exchequer, key players Michael Gove and Nigel Farage, and a host of other heavyweight names give their take in the Brook Lapping-produced doc, which is directed by Max Stern.

Johnson, who went on to become PM, was the first big name that Percy and her team got on board. It helped that he had just contributed to her Clash of the Superpowers: America vs China doc, also for the BBC, she recalls.

“I said ‘Our next [documentary] is on Brexit’. He just lit up and said, ‘Now that I could really give you an interview on.’ He said he’d give us ninety minutes, but he was there for almost three hours.”

The former PM surprised the team during filming with his account of deciding which side to back in the divisive Leave/Remain debate.

“We looked at each other in amazement when Boris said, ‘I didn’t do it to become Prime Minister, I would have been Prime Minister anyway.’”

She continues: “He really was wracked as to what to do, and in our interview, his hair got messier and messier for three hours. You really could feel him trying to make up his mind.”

Another key scene comes in the second episode. “There is an extraordinary sequence that starts with George Osborne, when he went out with his economic case and said that if we voted Leave, he would have to make a budget and that would leave everybody much worse off. He said all the things that absolutely terrified the backbench Tory MPs. He thought that would persuade them to go for Remain.

“It backfired, and within an hour, 50 of his own backbenchers signed a motion against him and he says ruefully, ‘That was the moment I lost my chance to be leader of the Conservative Party.’”

Percy’s approach is to reconstruct what happened inside the room. “Really, our questions are always simple. We have a chronology of key events, and we say, ‘At that meeting, what did you say and how did you reply,’” she says.

Contributors know Percy’s films often become the defining accounts of major events, which helps persuade them to take part. “You can say ‘It’s really worth taking the trouble to answer our rather detailed questions,’ because [the program] will last. It will be a historical document.”

Percy says “anyone who’s worth having says no at least three times.” The only notable absentee in the Brexit doc is Dominic Cummings, the influential Campaign Director of Vote Leave. It was not for want of trying. “The only time he actually answered was when Max [Stern] called him, and he said, ‘I’m sorry, I’m on the top of a mountain, I can’t talk to you right now.’”

Brook Lapping is part of Zinc Media Group. Zinc Distribution sells the series internationally and has licensed it to BBC Select for North America and RTÉ in Ireland among others.

A veteran filmmaker, Percy has a couple of projects in the works post-Brexit: A Very British Civil War. A definitive series on Donald Trump remains a long-term goal. “Trump is such a moving target. It makes it harder to do a series that will take about a year to make,” she says.

Percy is confident, however, that it will happen. “We’re going to get to Trump at some point, but you have to choose the right time. There is a moment when it’s not so raw that people are afraid to talk… and not so long ago that they’ve forgotten.”