I go the OPSI website when I want to look at recent legislation ... well, it beats mowing the lawn on Sundays, doesn't it?

www.opsi.gov.uk/

... the proof of wether or not its intended for the purpose of sexual arousal will lay with the judge!
That's exactly the way it should be, icey. Let the police and the Crown Prosecution Service say what they want about the images, it's up to the barrister at the bar to convince the judge that the accused is guilty of the crime by proving that the pictures come within the definition in the Act.

And of course, the defence will also want to put as many obstacles as they can in that barrister's way while he tries to make his case.

Then the learned judge will come to a decision. (And we all know what a bunch of pervs the judges are, don't we!)