"Child Pornography is narrowly defined in America in the sense that actual children have to be exploited in making it (i.e. no X-rated Simpsons parodies, for example)"
This isn't *yet* illegal in this country, but, believe me, it's coming!
See the "Coroners and Justice Bill" at http://www.publications.parliament.uk/pa/cm200809/cmbills/009/09009.25-31.html#j3_100a where: "(8)References to an image of a child include references to an image of an imaginary child."
In other words, we're going to be following Australia in that cartoon images of Lisa Simpson performing sexual acts will be illegal! (Where that leaves the London Olympics 2012 logo, I'm not sure!)
Also note that the authors of the Coroners and Justice Bill have cut-and-pasted big chunks straight out of the "extreme pornography" legislation in the Criminal Justice and Immigration Bill, presumably based on the principle that "well, we got away with it once, so let's do it again!"