I predict this won't go well for the government. It didn't happen in Australia, and I don't think it will happen in the UK. Plus, any government "filter" would be ridiculously easy to circumvent - in fact, that's one of the reasons the Australian government didn't go ahead with the filter - because it was easy, and legal, to use free tools to get around any filter.
I will say that people should criticize the UK government for considering this, but I will also say that it probably won't succeed.
Kids will always find ways to look at pornography. In the old days, kids used to steal magazines from their dad's closet or watch late night TV.
If you don't want your child to be negatively impacted by nudity and pornography, then actually PARENT them for a change. Buy an internet filter, Don't let them have a computer in their room until they're older, teach them proper rules and behaviour and keep an eye on them. It's not that hard. Again, as others have said, it's as if a lot of parents have thrown up their hands in despair, wailing "oh how can we protect the kids", instead of actually trying to protect their kids.
MY parents actually TAUGHT me stuff about rules and morals and social responsibility. MY parents actually limited TV and computer access til I was older. MY parents actually did a little something called PARENTING. And other parents can do the same. THEY didn't need no internet filter to raise me or my siblings, and we turned out great.