The porn situation in the UK is beyond fucked, As You Already Know (TM).
I mean for one, it's already fucked by the simple qualifier of showing female ejaculation (Hisasi loves his liquids. Fucking moisture everywhere). It has a few femdom chapters and even in other chapters interactions could be described as "humiliation" and "consensual verbal abuse".
All of which to say, if customs opens up your package and gives you a funny look, just snatch it from them and flip them the bird.
[note: All of this is unknown ventures as this is an erotic comic and not pornographic film. I am not a lawyer, I am not knowledgeable in UK Obscenity Laws, fuck it there's not even any explicitly stated teenagers in the book you're fine.]