I was on an adult dating site last year, and tried to access my messages via my 3 internet access on my mobile. When I couldnt get on I called 3 and was also told it needed to be unblocked, which they did over the phone for me.
When it still wouldn't work I contacted them again and after lots of blah, blah finally understood that if the network provider has their own adult content sites, such as porn, they will only allow access to these and block anything from other sites.
Don't know if that is the case with O2.