I'm British, I've never had ANY problems whatsoever understanding The Wire. I mean it's not even American slang per se anyway. If you grew up in South Carolina, you're gonna know Baltimore speak? Nah, I understand how most Americans understand, some fucking common sense.
I saw a British journalist talking about how he needed subtitiles. He poses as a pseudo intellectual. I say he's thick as shit if he can't make sense of the Wire.