We will check all your web traffic against a list that you can't see. If your destination IP is the same as for something on the list, we'll direct your traffic through a transparent proxy (which may or may not work properly). If what you're looking for is on the list, we'll just reset your TCP connection and not tell you why. We won't tell you because it's on the list, we won't tell you what's on the list or why. The only mention of anything like this on our website is a page you can only find by searching for "Internet Watch Foundation".
We get this list from a non-governmental organisation of which you have no oversight, there is no due process. If they don't like it, you don't get to see it. Anything they deem to be "racist" or "obscene" content can be blocked without your knowledge. This is anything "potentially illegal", not "illegal" because it's not necessarily been tried in a court of law.