I gotta be honest: I'd like to see what would happen if they tried this.
I assume you've got some evidence for your a claim that BT have broken the law like that?
Especially when changing the CNAME record is something that absolutely any DNS provider could be doing anyway...
Censoring wouldn't break the law, they're already doing that. And the blog post specifically mentions that it wasn't blocked using DNS but by a redirect from a Google server.