The IWF is the UK’s blacklist based filtering system for child-abuse material. It is voluntarily implemented by the Internet Service Providers (i.e. BT, Virgin etc) and is often held up as an example of how censorship can work.
However those who would hold the IWF as a shining light often neglect to mention it’s spotty history, or the concerns relating to due process;
The IWF reported a site to the UK Police for containing fictitious erotic material of an extreme nature, this led to the unsuccessful prosecution of the site operator (the prosecution offered no evidence)
In 2008, all UK users were blocked from editing Wikipedia articles after the IWF added a Wikipedia page to the blacklist, forcing all users to appear to originate from the same IP address. The IWF later rescinded the block (more info below).
In 2009 the IWF blocked the Internet Archive in it’s entirety despite their policy to only block the specific content. It was blamed on a ‘technical hitch’ but the content causing the issue never became publicly known
Concerns regarding due process have arisen because the IWF does not just block illegal content, it also blocks content which it believes may be illegal under UK law.
The blocking of Wikipedia is a prime example of the difficulties this introduces, the image blocked was the cover art from the 1976 album Virgin killer by the Scorpions. You can still legally buy the album, with that cover, from high-street stores and Internet music stores alike. The IWF, however, decided that the image was potentially illegal and without judicial oversight proceeded to block the relevant Wikipedia page.
Repeatedly criticised for blocking legal websites and not telling the website operator about the block, the IWF keeps it’s blacklist secret and is self-regulated which means there is absolutely no oversight over their actions.