A significant portion of British citizens are currently blocked from accessing the Chaos Computer Club's (CCC) website. On top of that, Vodafone customers are blocked from accessing the ticket sale to this year's Chaos Communication Congress (31C3). 
Since July 2013, a government-backed so-called opt out list censors the open internet. These internet filters, authorized by Prime Minister David Cameron, are implemented by UK’s major internet service providers (ISPs). Dubbed as the "Great Firewall of Britain", the lists block adult content as well as material related to alcohol, drugs, smoking, and even opinions deemed "extremist". 
Users can opt-out of censorship, or bypass it by technical means, but only a minority of users know how to bypass those filters. Accessing the server directly via http://126.96.36.199/ currently appears to work quite well, thereby rendering the censorship efforts useless.
Internet filters simply do not work, but leaving technical limitation aside, the CCC's example shows that unsolicited overblocking, meaning wrongly classified websites, is a common phenomenon in large censorship infrastructures. However, it may very well be that the CCC is considered "extremist" judged by British standards of freedom of speech.
"When these filters were introduced, their abuse was imminent. Today, we are shocked to learn that they not only block access to our site, but also to our conference," says CCC-spokesperson Dirk Engling. "We see this as proof that censorship infrastructure – no matter for which reasons it was set up, and no matter which country you are in – will always be abused for political reasons."