Given he wrote a PhD about masturbating to it I think this bar is met "The Act defines a ‘pornographic image’ as one which must reasonably be assumed to have been produced solely or principally for the purpose of sexual arousal."
The solely or principally part is a high bar with which the CPS has sometimes had difficulty. In this instance, the researcher's work seems to cover handily whether the drawings are "principally for the purpose of sexual arousal".
'...no matter how my research participants’ takes on shota differed in terms of favourite theme, preferred age and style of characters, how they related their own selves to the story, and so on, they had one thing in common: almost all of them said that they masturbated to shota material...'
I am surprised - but not as surprised as I should be - that the Manchester University Research Ethics Committee (or in this case likely a small subset of the UREC who reviewed the application) did not point out to the student that there was a risk of his research being used in criminal evidence against him.