I gave a scathing comment last time about the ramifications of opt-out censorship, but in my examples of it working properly opt-in was singled out as being ideal.
FYI, this opt-in filter has been around a few years, I dislike that it also filters explicitly pornographic sites unless you call in (yes, I had o2, yes, I tried) but that seems to have been the norm with a few cellular phone providers (such as T-Mobile).
the thinking is probably along the lines of: "We give sim cards away for free, we don't know who will use them, internet costs very little, we should probably stop the worst stuff just in case of complaints"; contracts are instantly unblocked and the same goes for broadband (because getting a contract means you must be over 18 anyway), at that point it's the responsibility of the contract holder to filter internet for any under-age people on the line.
well, that's how it was, the way it's going is... slightly different.