Censorship is a slippery slope. The United Kingdom: my home. Case in point.

Zack Whittaker/CNET

Nearly five years ago, the U.K. flipped the Web censorship switch. Most U.K. residents didn't even notice. Designed by telecommunications giant British Telecom (BT), "Cleanfeed" was used to filter out child abuse imagery, and it did so with great success.

Users would not see a notice or a startling warning about the content they had inadvertently accessed or had tried to reach. The page just wouldn't load.

In 2007, Home Office minister Vernon Coaker ordered all U.K. ISPs to subscribe to Cleanfeed to prevent access to scenes of sexual abuse and "criminally obscene" content.

And then things began to change.

The U.K.'s antipiracy legislation, the Digital Economy Act, was brought into law by a tiny minority of parliamentary representatives in 2010. In all fairness, it was only a matter of time.

But a series of delays means the law has yet to swing into full effect. Its "three-strikes" system, designed to inform copyright infringers that repeated acts would lead to Internet disconnections, has been put on ice until 2014.

The United States followed in the U.K.'s footsteps with the Stop Online Piracy Act, also known as SOPA. In a similar fashion to the Digital Economy Act, it would allow copyright holders to seek court injunctions against Web sites that "enable or facilitate copyright infringement."

On January 18, no more than three months after the bill was introduced to the U.S. House of Representatives, the Web went dark for a day in protest. Dozens of prominent Web sites temporarily shut down or displayed public protests in response to the 'censorship' bill. SOPA was shelved two days later.

The "enabling" of the U.K.'s Digital Economy Act was not quick enough for copyright holders, like the music and film industry. Enough was enough, and they brought a case to court.

In April 2011, the High Court in London ruled BT must block access to file-sharing site Newzbin2 at ISP level -- using none other than the Cleanfeed system. It was widely seen as a "test case" building up to forcing bigger file-sharing sites off the British Web.

BT did not appeal the decision, leading to a damaging legal precedent under U.K. law. From the moment of that ruling, any individual or organization could bring a separate case against any other online property.

And so it began.

Almost exactly a year later, six of the U.K.'s largest broadband providers were told by the same High Court to impose a block on their customers from accessing magnet-link sharing site The Pirate Bay.

The numbers breakdown shows how many people are affected as a percentage of the wider U.K. population, but perhaps more worryingly, how many are affected when compared with the overall number of U.K. broadband users.

Virgin Media was the first ISP to block its 4.3 million customers from accessing The Pirate Bay, on May 2, only days after the ruling. A week later, Orange Broadband's 713,000 customers found their access was cut off. On May 30, Sky Broadband imposed a block for its 3.8 million customers.

O2 Broadband was next, on June 7. Despite not being named in the judgement, Be Broadband customers were also hit by the block because Be was bought by O2 in 2006 for 50 million pounds ($27.2 million).

An O2 representative said its total subscribers base -- of both O2 and Be users -- stands at 617,000 customers.

TalkTalk's 4.1 million customers were next. It was the most recent -- and most rebellious -- in how it blocked its customers from accessing The Pirate Bay.

In a daring stunt, it was the only ISP to tell its users when they attempted to access the site that the British Phonographic Industry, the British record industry's trade association that brought the case to the High Court in the first place, was to blame for the block.

All named ISPs bar one have imposed blocks. BT's 6.3 million customers have not yet been blocked from accessing the "pirate" Web site. The telecommunications and broadband company -- the largest in the U.K. -- has been given more time to consider its options.

However, the High Court told ISPs to block certain IP addresses, Web site addresses, and domain names. The Pirate Bay was quick to change its Web site IP address -- circumventing the block completely -- and many other Web sites offered proxies to allow access to the site.

And here's the kicker: at no point were U.K. citizens barred from circumventing the blocks to access the "pirate" Web site.

By the time all the blocks are in place, the total tally could reach as many as 20 million users, which amounts to about a third of the U.K. population.

But the effect of the widespread block is far more concentrated.

The six ISPs forced to block access to The Pirate Bay serve more than 92 percent of the U.K.'s broadband customers. The figures are a bit sketchy and haven't been updated by Ofcom, the U.K. communications regulator, since the third-quarter of 2011.

Ofcom said there are more than 20 million residential and small business lines in the country as of February. We're now in June, and the figure would have risen slightly. It's fair to say that the very vast majority of U.K. broadband users will find their Web freedom restricted as a result of the High Court ruling.

One has to hand it to the BPI. It brought a highly sophisticated, targeted court case against U.K. broadband customers.

After all, it seems it was the ordinary broadband user who was in the BPI's crosshairs, not The Pirate Bay itself.

Update (June 20, 13:35 BST): All ISPs named in the April 2012 court order have now blocked access to The Pirate Bay following BT's move to enable the block on June 19.