Sky's filtering is now opt-out:

> What we’re doing now is simply making sure that the automatic position of Sky Broadband Shield is the safest one for all – that’s ‘on’, unless customers choose otherwise.

src: https://corporate.sky.com/media-centre/our-blog/2015/sky-bro...

When the whole opt-in opt-out thing hit the news I remember all the press was insisting it was opt-in. Simply insisting, then basing their arguments off that.

Not mandatory though, that's most the important thing.