>Anyone who uses a 'best interests of the children' argument
>should be immediately shipped to an island populated entirely
>by other people just like them ... Put them all on the island,
> setup cameras,
The island is called Great Britain, do feel free to visit us. Everyone over the age of about, oh, five, has a mobile phone. 3G mobile data and fibreoptic broadband has near-complete coverage in all of the island's urban and suburban areas, with rollout plans for all rural areas except the Scottish Highlands. We also have CCTV cameras covering pretty much every urban area and all major roads on the island.
British people value "doing the right thing" above freedom. Freedom includes the freedom of the strong to persecute the weak, and we don't think that that's the right thing.
We Brits have an extremely conservative attitude to child safety. For example, any adult who visits a school during school hours more than twice a year, is required to undergo a background check. Missing child investigations instantly lock-down nearby borders- try to board a ferry to the European continent with your young family during a missing child alert, and you can guarantee you'll questioned and checked quite thoroughly (been there done that, especially when my youngest matched the missing child description).
You may not like filter-by-default, but it is, at least, consistent with our other national child safety policies. We are not the USA, and whilst we are friends, we don't always think the USA does the right thing.
We rate fairness above freedom. It's fair to filter all mobile data by default, if and only if, it is very easy for an adult to turn that filter off. That provision makes it fair, makes it British.
Seriously, I'm with GiffGaff and turning the filter off/on is one checkbox on a webpage, and that webpage isn't difficult to find (log in to GiffGaff's website; click Phone Settings, job done). There is no human interaction. It's monumentally simple. It's on the same web page as the checkbox to turn billing notification off (by default, GiffGaff text you after every call, telling you your balance). I turned it off, no problem; it covers all 18+ services such as betting too.
I found the filter made it an easier decision to me when deciding whether to give my eldest daughter a smart device. Sure, I could have set the DNS to OpenDNS Family Filter (which is what I did with her Linux laptop, and frankly I think all shop-bought PCs sold with operating systems should have that by default) but this setting on GiffGaff just made my life easier.
The problem with setting DNS on a smart device, compared with a laptop, is that there is no concept of sudo on Android, and a pretty poor implementation of admin rights. Any user (or permissioned app, for that matter) can change the DNS. So having a service-provider-level filter is quite handy for smart devices in a country where 95% of kids own one long before the age of ten.
When she's old enough to take on the account's bills, she can decide whether to turn GiffGaff's filter off on her smart device. And when she's experienced enough to be entrusted with sudo she can reset her laptop's domain name servers to whatever she likes.
Don't get me wrong, it would be better for Android et al to introduce proper superuser-based security. But until the vast majority of them do that, provider-level filtering remains consistent with child safety law in England and Wales.
 There's another Petit Bretagne - Little Britain - in what is now north-western France. They're descended from Cornish Celts and speak a dialect of Welsh. "Great Britain" in this context just means "the big island", not any statement of superiority.