There are moments, when The Devil's Whore, coming soon on Channel 4, seems almost too rich and exotic to be a British drama. Hollywood's accepted agenda for heroes – whether fighting aliens in Independence Day, Persians in 300, slave-owners in Amistad or the British in pretty much anything starring Mel Gibson – in the struggle for liberty and justice. Our homegrown screen champions, conversely, usually shuffle awkwardly through deeds performed for money, deception, loyalty or pretty compromise. So it's strange to hear epic speeches against tyranny delivered, without irony, by English lips.

"My liberty is his to take – but not to give," a Leveller cries. "I am freeborn John Lilburne. We will not live like slaves. Nor will we loll in our beds while he bring in an Irish army or Scotch army to kill us." Later, Oliver Cromwell pleads for Lilburne in Parliaments thus: "Then where is the justice for John Lilburne that rots still in the Fleet by a sentence most illegal, against the liberty of the subject, bloody, wicked, barbarous and tyrannical?"

The Devil's Whore recounts the stories of comrades, enemies and lovers who battle, with varying degrees of idealism and brutality, through the civil war, from the closing days of the Eleven Years' Tyranny to the post-war manoeuvring between parliament's factions as the army, the radicals and the conservatives wrestle for the nation.

As in Our Friends in the North (Peter Flannery's previous big hit), which saw Daniel Craig, Gina McKee, Christopher Eccleston and Mark Strong sharing a screen, the cream of our television acting talent has been assembled for this F7m four-parter. John Simm plays a feral mercenary called Sexby; Dominic West is Cromwell; Michael Fassbender is Thomas Rainsborough, and Peter Capaldi transmutes the splenetic aggression of his The Trick of it spin doctor into Charles I's stutter. At its heart is the Devil's Whore herself – Angelica Fanshawe, played by Andrea Riseborough, of Margaret Thatcher in the Long Walk to Finchley fame.

Fanshawe is the one truly fictional creation of Flannery and his co-creator, the historian Martine Brant, in the sea of real historical characters. She was conjured by Brant out of a document called The Commons of Newgate - essentially gallows confessions by 17th-century women, gathered by a priest at the infamous prison. "Most of these women broke the mould, didn't conform and were either pitifully misunderstood or persecuted," Brant says. Angelica is nobleborn, but driven by passion and sensuality. "She is a really open and giving and tactile person," Riseborough explains during filming in South Africa. "She is not of her time, because the decorum of court stifles her" – she breaks off to demonstrate the accepted walk for nobles at court, tottering like a tumbling doll. "That's so wrong for her, because she's instinctive and sensual and led by her heart."

As a result, Angelica rejects God when her mother deserts her, but then finds Harry – her cousin, friend and husband – rejecting her for intellectual and sexual excitability. At the start of the war she is the king's favourite, but gradually her encounters with Levellers such as Thomas Rainsborough (Michael Fassbender) and Simm's cynical Edward Sexby radicalize her. She strides through 20 years of chaos, open and hungry for knowledge and experience as the battle for democracy is won around her.

It's won in impressive style, too. The South African location brought huge cost-saving that meant that great civil-war such as 1642's Edgehill and Cromwell's attacks on cathedrals and stately homes look like grown-up battles rather than handful of extras and smoke bomb. The producers also found an area in the Cape where the countryside looks similar to England, complete with oak trees. Most of the key moments feature – the forming of the New Model Army in 1645, the execution of Charles in 1649 – but smaller historical details are expanded for dramatic effect. Simm's Sexby, for instance, is based on real Leveller who plotted but failed to assassinate Cromwell. Te's a footnote in history books, but a crucial player here.

"I'm not selling this as a way of passing your GCSE in 17th-century history," Flannery acknowledges. "The Devil's Whore absolutely takes liberties with historical fact and historical character – I'm trying to write a thrilling story about those times and what the issues were. To do that, I have to make merry with events and characters a little."

All the same, the list of research documents Brant and Flannery consulted stretched from The Common of Newgate through academic and popular histories of the era, and Pauline Gregg's biographies of Cromwell, Charles I and Lilburne, to the original pamphlets issued with radical fervor by the Diggers and the Levellers – proto-socialist movements of the time. In a first for primetime drama, however, Flannery's key source was the Bible.

"The book I returned to again and again was the King James version, since it was written in 1604, with contributions from brilliant poets including – possibly – Shakespeare," he explains. "I picked it apart to get the language and the tone of the dialogue. It's full of muscular metaphor and simile – exactly the feel I wanted for the characters."

John Simm addressing the camera along the blade of a knife as he prepares for war... Charles I shot from a vast distance as he peers through a dark window in a white wall to watch an execution...
Indeed, with all these ingredients, The Devil's Whore is easily the greatest civil-war drama we have seen since 1983's By the Sword Divided.

Astonishingly, since By the Sword Divided, there have been eight Jane Austen adaptations and countless episodes of Sharpe and Hornblower, as well as adaptations of War and Peace and Vanity Fair, recounting aspects of the Napoleonic wars from the domestic to the global. The civil war, the foundation of our democracy, has had a tea-time Children of the New Forest.

In part, this is a result of the paucity of civil-war fiction. In September, when the Royal Court Theatre hosted a reading of Caryl Churchill's civil-war play Light Shining in Buckinghamshire, readers of the literary website Ready Steady Book muster only seven substantial works of fiction about the era, and one of those was Paradise Lost.

"I can't for the life of me understand why we're not more proud of civil war," Flannery argues. "In our schools, we're taught that the Cavaliers wore their hair short and banned Christmas. And that's about it. Regicide, interregnum and restoration, and there you have it. We've been invited to feel that England lost its wits for a little while, courtesy of some rather dour men. Actually, it's the crucible for all the subsequent European revolutions. It radicalized a lot of people and left a legacy of ideas that we're still battling over. We should, as a nation, be informed about that and take proud interest in it. Also, for a writer, it's the most glorious landscape for love and action."

So persuasive is Flannery's view that it turned the Irish Catholic head of the actor Dominic West when he put on Cromwell's helmet. "I came to Cromwell not particularly loving him," West says with the beginning grin. "I knew he ransacked Ireland, I knew he said 'warts and all', and that he cancelled Christmas. I suppose you are always in danger, as an actor, of giving a character the benefit of the doubt, and that's what I'm doing with Cromwell, in that now I really admire him. I think he did everything for an honourable reason. Even the Irish thing was really a 17th-century Hiroshima and Nagasaki. It was a horrific as that, but it was done so that, by hitting southern Ireland and Wexford hard, the war would end sooner. He was always in favour of the king, and he didn’t want to get rid of him until he realized the war wasn't going to end. But he wanted him to be king without power – which is what we have now, really. This is what he created."