The makers of this movie think that slightly roughing up suspects in interrogation is what dirty cops do. Here in America, our dirty cops shoot innocent people in broad daylight for no freaking reason and somehow inexplicably avoid prison. This movie thinking it is edgy is hilarious.
I'm not defending the film - it's terrible - but the emulation of American culture is hardly something to aspire to. You're welcome to your gun culture.
They're not really dirty though. That's the point. They're a little rough, but only enough to get the job done. They're not supposed to be corrupt or anything. The former Yoof-turned-cop is the prime example.
They are not supposed to be dirty cops in the true sense, they are just bending the rules a bit to get the job done. Its like Dirty Harry, he wasn't really dirty was he, he was honest and straight but he sometimes had to bend rules to get the job done and he never hurt anyone who didn't have it coming, at least not on purpose anyway. The UK and USA are different cultures so you cant compare them both. What the cops did in this film was quite off the wall and dirty compared to how we would normally expect cops in the UK to act but as you say it wouldn't be seen as such by US standards.
Dirty Harry bends the rules but Regan in the movie was dirty. He stole some of the Kruggerands at the beginning of the movie. Regan in the tv series was never bent.