It seems to differ by provider. When I was with Three it was an irritating process of having to either call up or visit a shop in person and say "I want to look at the naughty pages, please". Another provider (I can't remember which) had a method where you had to supply a credit card number.

I'm with "1p Mobile" now who are a virtual network on EE, and their adult content block is just a toggle in your online account, with no faffing around required - you can just hit the toggle. I presume the idea is that you don't give little Timmy the password to his own account portal, but I don't know what's to stop him getting his own SIM by himself.

With Three, I found the adult content block caused other problems with SSH connections dropping, various random stuff getting blocked and so on, which all went away as soon as I had it disabled, so it's worth doing even for non porn fans.