The movie, from the director of Love Actually, Richard Curtis, is about young people, the 1960s and pop music.
It centres around a band of DJs who captivate Britain by playing music that defines a generation as pirate radio plays rock and pop from the high seas 24 hours a day.
Those behind the film said that more than half the population of Britain listened to the pirates every single day.
Arterton described the film as “fun, fun, fun all the time. It was brilliant”.
Branagh said: “There was a lot of fun from being the guy who is trying to shut them down.”
Nighy said: “I did enjoy making the film and I’m not just saying that.
“We floated on a boat off the coast of Dorset with a bunch of very cool guys, we played lots of music, you could have a toasted cheese sandwich any time you wanted.”
Nighy said he had never been a classical English actor, adding that Shakespeare posed a problem “because you can’t operate in those kind of trousers”.
Ifans, wearing rock star dark glasses and a snakeskin-style jacket, ambled up and down the red carpet.
Curtis said that Notting Hill star Ifans was handsome in this film. “He’s improving with age,” he said.
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