‘Rape porn’ possession to be punished by three years in jail, David Cameron to announce

[BainsBane]

235. You aren't dealing with common sense

Your examples are absurd. There is no similarity between snuff and Saving Private Ryan.

Now you're talking about proof, which is again a matter for British law. Presumably the crown would still bear the burden of proof, but again I don't claim knowledge of the British legal system.
I seriously doubt that anyone would dispute possession of rape or child porn, like any other crime under British law, should be proved rather than assumed.

Most of the discussion here has concentrated over whether banning rape porn is a good idea. I think it is, though I don't know if it would even be possible under US law. The reason I think it is a good idea is simple: rape is bad. And nobody is going to mistake a pornographic depiction of rape or snuff for Saving Private Ryan. Courts can and do make distinctions between drama, art, and porn all the time.