Yeah, she was over 18 at the time of filming but... The violent porn law is so vague that it allows virtually any sexualized violence(including stuff that has been passed by the bbfc as 18) that the CPS takes a dim view of to be labeled "Porn", regardless of what anyone in their right mind would consider "porn", they use the word porn for any image/clip that they 'think' is used to arouse sexual thoughts, so anything that includes nudity, sexual violence of a graphic nature or intent can lead to a visit by the old bill. As a lawyer said at the time the law was passed "If you and your partner film yourselves spanking each other fine, but post that on the Internet and this law allows you to be arrested and prosecuted for it" Though I can't imagine any UK jury convicting anybody under this stupid law, it's best to be aware of the powers the Feds can level against you, and if the bbfc have censored a scene that would give the CPS more ammo to go after those who post online, paranoid or not hate to see Shameless make headlines for all the wrong reasons. And that razor scene seems right on the mark, imagine if you were just an average civil servant type with no interest in movies working for the CPS and got shown that scene, chances are you'd hit the panic button just to be sure. Of course they might look at it and not give a s**t
... but you never know.