"Shut it, love, actually, or I’ll hole-punch your face.” “Kiss my sweaty balls, you fat f***.” “You sounded like a f***ing Nazi Julie Andrews.” It doesn’t take long for In the Loop to carpet-bomb you into willing submission. With the laughs detonating at a rate of far more than one a minute, the script comes at you like a block entry for The Oxford Dictionary of Weapons-Grade Invective.

The source for this flurry of slurry, offered in the bilious spirit of great Augustan satire, is, of course, none other than Malcolm Tucker. Viewers first came across the PM’s potty-mouthed communications chief in the BBC comedy The Thick of It. In this X-rated update of Yes Minister, he pistol-whipped all those unfortunate enough to cross his path: spluttering ministers, quivering civil servants, squirming journalists and quaking press spokespeople. With In the Loop, Tucker now has his own film.

Yet in an intriguing shift, on the bigger stage, Tucker is no longer the biggest bully. The British government is all for joining America in a morally dubious war in the Middle East. It is Tucker’s task to present the Department of State with the compelling evidence that will secure a UN resolution. When he gets to Washington, however, he is snarled up in a turf war between hawks and doves. His task is made harder by the presence of a pathetically biddable young minister who keeps failing to toe the party line that war is “neither foreseeable nor unforeseeable”. In short, a remarkable thing happens in In the Loop. You actually feel a flicker of sympathy for the fictional counterpart of Alastair Campbell.

Despite the presence of a wonderfully talented American cast and, in James Gandolfini, one copper-bottomed star, the film is entirely British. Its presiding geniuses are in fact Scottish or, to be fully precise, Scots of Italian ancestry.

Armando Iannucci and Peter Capaldi sit in Iannucci’s lair at Television Centre, in what feels like a sixth-floor Portakabin. They give off the contained air of tacit satisfaction. On their hands they have a palpable hit, but they can’t jump up and down about it yet. For the moment, all they have to go on is a single screening at Sundance in which, Iannucci says, “people were (a) saying they wanted to go and see it again, and (b) quoting their favourite line”. There are a lot to choose from. And sight gags galore. Capaldi spends half our conversation in fits as they are recalled.

It is perhaps making too bold a claim to say that In the Loop will enlarge our understanding of exactly how Britain ended up in Iraq, but with further evidence published this month that the dodgy dossier was indeed sexed up, it’s worth remembering that Whitehall insiders have admitted to Iannucci that he had them bang to rights in The Thick of It. And they are enamoured. Why else would he have been granted permission to film In the Loop at No 10? It seems mind-boggling. “They said, ‘Fine, you can film here any time,’ ” Iannucci recalls. “We ended up in the cabinet room. I think it’s the flattery of the fact that we do them, irrespective of what we say about them. All the Malcolms and Jamies [Jamie, played by Paul Higgins, is another Gorbals press gauleiter] had brought their cameras because they wanted to take photos of Peter.” “It was a bit like going to a Star Trek convention,” Capaldi adds.

Washington turned out to be no less starry-eyed. On a fact-finding mission, Iannucci found himself speaking to Joe Biden’s chief of staff. “And he was saying, ‘One of the pleasures of this job is, every now and then, you get to meet interesting people. I mean, I was so excited last week when we had a reception and Bradley Whitford [who plays Josh Lyman] from The West Wing came.’ And I’m just thinking, ‘But you are him. You are him.’ They forget what their position is.”

The Thick of It has been shown on BBC America. Director and star had an inkling of how Tucker’s sinful charisma can work on the unlikeliest of psyches when Iannucci’s Washington insider and unofficial adviser asked Capaldi to leave an abusive message, as Tucker, on a friend’s voicemail. The friend was a soldier who had been injured in Iraq. “He was a big fan of Malcolm,” Capaldi says. “Then, later, his friend phoned back and left me an abusive message. What was great was he was Puerto Rican, but he was doing a Scottish accent.”

It is taken as read that Tucker is Alastair Campbell. “In fact, it was vague when we started,” Capaldi says. “Clearly, he’s a spin doctor, so Alastair Campbell is one of the key references. But even when I did it originally, I didn’t know how he spoke and I didn’t know how he would perform at full flood, so we were making all of that stuff up.”

As if to clear up any doubt in the matter, Campbell seems to recognise his portrait. He even invited Capaldi to interview him in character. “I had to explain that I’m not Malcolm, and would need six writers to do that.” “He’s not his best spin doctor, is he?” Iannucci adds. “I think he was confusing fact with fiction. And not for the first time.”

What Whitehall will make of this latest portrayal remains to be seen. The joke is that the Special Relationship is a joke. This came from Iannucci’s wanderings around Washington’s corridors of power. “It’s just that awareness that we are nothing. A lot of people said, ‘Hey, we sure screwed that Tony Blair.’ They had not yet been able to work out why he was so amenable.”

Among the hilarious riffs on this idea is a summit in Westminster where the American delegation is so huge, the Brits have to pack the meeting so as not to look outnumbered. The silent extras are known, in Iannucci’s glorious euphemism, as “room meat”. (Someone in Washington has already told Iannucci he “must start using that phrase, because it just works”.) To get that wide-eyed but hurt look on the faces of his British cast in America, Iannucci asked them to “try to think of the first time you ever went out to LA to meet people in Hollywood, and how excited you were, but, in the end, how disappointed you were that nothing came of it”.

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