Apols for this missive ...
The main problem authorities have in terms of prosecution is that they have to prove the person made the images (as opposed to just finding them somewhere online and re-posting them). Secondly, the prosecution has to prove that the images were made with the intention of harassing or humiliating the person involved. Thirdly, the prosecution has to prove that the person making the images was doing so for sexual gratification (as opposed to just doing it for a laugh).
For example, posting an image of Holly Willoughy showing her knickers for shits and giggles isn't going to cross the theshhold of illegality. Sending that image to her personally and saying: "I'm jerking off to this image of you" could potentially get you into legal issues.
That's my interpretation of current UK law. Ps. Don't ask me to be your barrister.