According to The Register (IWF confirms Wayback Machine porn blacklisting), explanation of the loss of access to the internet archive to customers of some UK ISPs is due to an IWF blacklist, containing URLs contained within that archive.
The Register asked the IWF what URLs were blacklisted, who at the ISPs were responsible for implementing the blacklist, and why ISPs were blocking the whole archive, but the IWF refuse to comment on the URLs on the blacklist (it's their policy), and refused to (or were unable to) answer the other questions.
This does raise a number of objections to the use of internet filtering to "protect" the population - as the list of URLs grows, the operatives at ISPs who are tasked with implementing the filters will tend to take short cuts and block large chunks of the internet.
However, we still don't have any access to the process by which web pages are judged illegal. I note from the Register article that the IWF says
... it has taken action in relation to content on www.archive.org involving indecent images of children which contravenes UK law (Protection of Children Act 1978). The URL(s) in question were added to our URL list according to IWF procedures
This is, or course an advance on the public statements at the time of the last IWF debacle, where a ban on a 1970s LP sleeve removed Wikipedia access from many UK ISP customers. Then a panel of four individuals, trainined by police, judged web pages as to whether they were "potentially" illegal. One item has remained the same, the archive pages all contain images of child porn - the IWF, despite its wider-ranging remit, seems only to be concerned with child porn images. The affairraises further questions, not only on whether our internet content should be filtered, but also on the means by which this filtering is implemented.