It turns out that at least one of the ISPs in question has been broadcasting fake BGP routes for blocked sites, in order to force downstream ISPs to use their blocking proxy. There's one big hole in their plan - they didn't give any of the ISPs in question access to the proxy, so all users got was a blank HTML page for any Wikipedia content. Oh, and they didn't actually tell anyone that they were doing this. You should have seen the messages filling up the support forums.
It took my ISP 48 hours to figure out exactly what was going on and fix it.
Back in the early days of electronic civil liberties the cipherpunks had a saying "The Internet treats censorship as damage, and routes around it." Well, we're now routing around the damage this censoring proxy, just the way the Internet is meant to work
Which basically boils it down to: IWF FAIL.
I'm actually astounded by this action on their part. I was running an ISP when the IWF was founded, and the folk running it them would never have stooped this low, especially to block what is legitimate content and which is hosted outside the UK. This action is not proportionate, and certainly not what the IWF was set up to do.