Re: Windows Server
Cleanfeed is just BT's implementation. It has become something of a genericised trademark though, as other ISP filters are sometimes referred to as Cleanfeed.
There is no government mandate on exactly how the filtering is to be achieved. There isn't even a statutory requirement that filtering be used at all - but there have been a few political statements making it quite clear that if all major ISPs do not voluntarily maintain some form of filter, a law to force them will be passed. It's an optioinal-but-not-really thing.
As for how it works, who knows? The systems are very secretive. There was the incident some years ago in which part of Wikipedia was blocked that shed some light on how it works, and the Australian block list was leaked once revealing it to be full of mistakes and over-blocking, but for the most part it's so secretive that when a page is blocked some ISPs spoof a 404 error in order to obscure that any filtering is happening at all. The Virgin Media system, as of 2008, used a combination of DNS filtering, IP filtering and - for hosts which required blocking only some files, or shared hosts - redirecting all HTTP traffic for a certain host through a proxy server which could block specific files. That's how the Wikipedia block was detected - for a brief time all UK traffic was being directed through just a few proxy servers, and appearing to come from this handful of IP addresses, which played hell with Wikipedia's abuse and spam detection. That was more than ten years ago, so it's very likely they have moved on to something more advanced now - probably involving a list of suspect IPs on which to carry out DPI.