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.

[1] https://wiki.openrightsgroup.org/wiki/Internet_censorship

[3] https://en.wikipedia.org/wiki/Child_abuse_image_content_list...

[2] https://wiki.openrightsgroup.org/wiki/Cleanfeed#cite_note-LI...