There seems to be a bit of confusion here. There are at least three quite separate things being proposed:
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Rape porn being made illegal
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Greater measures to catch and deter child abusers sharing images etc. online
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ISP filters to block legal porn
There is no technical problem at all with making rape porn illegal. What is being proposed is an additional clause to S.63 of the 2008 CJA.
From Wikipedia:
Section 63 of the Criminal Justice and Immigration Act 2008 is a law in the United Kingdom criminalising possession of what it refers to as "extreme pornographic images". The law was enacted from 26 January 2009. It refers to pornography (defined as an image "of such a nature that it must reasonably be assumed to have been produced solely or principally for the purpose of sexual arousal") which is "grossly offensive, disgusting or otherwise of an obscene character" and portrays "in an explicit and realistic way" any of the following:
-An act threatening a person's life
-An act which results (or is likely to result) in serious injury to a person's anus, breasts or genitals
-An act which involves (or appears to involve) sexual interference with a human corpse
-A person performing (or appearing to perform) an act of intercourse (or oral sex) with an animal (whether dead or alive)
and a reasonable person looking at the image would think that any such person (or animal) was real.
The term covers staged acts, and applies whether or not the participants consent. Classified works are exempt, but an extract from a classified work (if the image was extracted for the purpose of sexual arousal) would not be exempt. Whether an image is "pornographic" or not is up to the magistrate (or jury) to determine by looking at the image; it is not a question of the intentions of those who produced the image. If an image is held in a person's possession as part of a series of images, the question of whether it is pornographic is also determined by the context in which it appears. Therefore an image might be legal in some contexts but not others. Serious injury is not defined by the act, but is up to the magistrate or jury. Guidance on the bill gives examples of acts which would be covered: depictions of hanging, suffocation, or sexual assault involving a threat with a weapon; the insertion of sharp objects into (or the mutilation of) breasts or genitals.
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So what is being proposed is to add pornographic depictions of rape to those other four. This won't affect Rape Crisis's website, lad's mags, GoT or anything else which isn't rape porn.
There are massive technical problems policing it, as there are with child abuse images, but not with making it illegal.
ISP filtering of legal internet porn is a whole separate issue which is already being discussed on several other threads. It is riddled with problems (and yes they are technical, sorry) and it's my belief that it will leave us worse off than we are now in terms of protecting children. Kaloki/Murder's blog explains these problems in simple language for those who want to understand.