The article somewhat glosses over it, but you can buy a PASS age verification card at the local post office for 15£. That one is widely accepted and it doesn’t contain unnecessary information that might cause trouble if it leaked (like for example a passport does). And 1 in 3 adults (according to the article) have an Apple account that’s old enough so that they will automatically be unlocked, no further documents needed.
The article strongly accuses iOS of being a walled garden, but I don’t see that as a particularly strong argument after iOS being locked down for ~20 years now.
And as a parent, I know that if child protection is opt-in, there’ll be a huge fight about it, because some other parents won’t activate it, which then makes the situation unfair for the kids. I’d much rather have it on by default so that all kids are treated the same.