>>varisp+K81
These are optional adult filters that you can unblock. I have phones on O2 and EE and am able to access archive.org. I believe that you do need to have proven to the ISP that you are over 18. Paying by credit card counts.
The government requires that ISPs provide some opt-out adult filtering, but archive.org being blocked seems like an over-zealous third party moderation list, not government censorship.