Here's a flowchart showing how Cleanfeed — the secret British national firewall that is presently restricting access to Wikipedia – operates:
Our routers firstly check the IP address of the server that's hosting the URL you're trying to access. If they determine that the IP address is also used to host one of the websites on the IWF list then your request is passed to the IWF proxies.
A lookup is then done and if the address you're trying to access matches one on the list then the request is denied.
Translation: a third party now monitors every request made to Wikipedia from the six ISPs that participate in the Great Firewall of Britain.