More interestingly, O2 has not blocked access to its own site. That would be silly. It has, however, blocked access to http://www.giffgaff.co.uk, one of its competitors.
I've seen it claimed that it's blocked access to the Conservative Party's web site. Based on all this, that's a block I'd like to see remain in place.
That will almost certainly be O2's whitelist opt-in blocklist option, which only allows access to sites that have been explicitly flagged as safe. This is not the blocklist that you'd be signed up to as the default option.
I've seen it claimed that it's blocked access to the Conservative Party's web site. Based on all this, that's a block I'd like to see remain in place.