No, there is no "poster child" to justify wider site-blocking, nor even narrower site blocking. It is illegitimate governments to manipulate Internet nodes so as to filter what passes through them. If they have a problem with some website, let them take it up with the publisher/creator.

If the UK government is worried about children exposed to harmful content, well, let us first remember that they are assisting and supporting the massacre of children in Gaza, and the starvation of hundreds of thousands of them. And when that stops - I suppose they're welcome to suggest content filtering software to parents.