A side-effect was that users of those Internet Service Providers in the United Kingdom were not able to modify other Wikipedia articles anonymously anymore.

It did not take long however before some users created a Greasemonkey script Antiproxy 4 Wikipedia so that users from the United Kingdom who cannot edit Wikipedia entries anonymously anymore have the opportunity to do so. A Firefox extension based on that Greasemonkey script has been designed in the meanwhile which can also be used to edit articles anonymously.

A more technical explanation can be found at Computerworld:

The measures applied caused the ISPs to redirect traffic for a significant portion of the UK’s Internet population through six proxy servers, which can log and filter the content that is available to the end user, Wikinews say. But the knock on effect resulted in Wikipedia being unable to distinguish UK users from one another by their IP address. That triggered Wikipedia’s anti-abuse mechanism, blocking all non-registered UK users from editing articles, according to the foundation.

A search for Virgin Killer will reveal the album cover on many websites who are still available for UK users.

Update: The Firefox extension has been taken down, it is no longer available on the Mozilla website. You can still use the userscript.