I’ve actually just been reading up on the UK law, and in regards to written content, child pornography laws don’t apply. Child pornography laws apply specifically to any kind of drawn/depicted/photographed/filmed image, not written material.
What it does fall under is Obscene publications, however it would be up to a court to decide if any given publication was classed as obscene, and likely to ‘deprave and corrupt’ any individual likely to come in to contact with the material.