I have never heard of the self-signed certificate, that would be interesting to report to the Open Rights Group [1]
The filtering in the UK is by inspecting HTTP requests, so when a single image on wikipedia.org was blocked, every request to Wikipedia ended up going through each ISPs hidden proxy. [3]
According to [2], HTTPS sites aren't filtered -- but it references a page from 2004. I suspect HTTPS sites are now simply blocked outright at either DNS or IP level, but I don't have a way to verify this, and can't find any details.
Correct me if I'm wrong but I'm pretty sure this enables complete MITM by Vodafone when using cellular network.
http://imgur.com/b0il5xb http://imgur.com/3mw5ZGZ http://imgur.com/6ehhfuZ