Think of the children?
It is not clear exactly what the agenda is here. If this is just a matter of "think of the children" then clearly, as said by the culture secretary in September "keeping children safe online should be the responsibility of parents, not ISPs". There are already ways for parents to configure parental controls on their computers as well as free and paid applications and services that offer lots of control for parents. There is no problem to solve. ISPs are there to provide communications, to shift packets, not look at the content.
Nanny knows best?
So, we have to assume that the agenda is more "nanny state". The government thinking that British citizens are not mature enough to make their own decisions, and that the state has to protect us. I find that attitude somewhat insulting, personally.
I am assuming here that we are talking about legal content that is expressing extreme views somehow, as if it was actually illegal it should be taken down at source.
Let's burn books
What is "harmful content"? Information is information, and is not itself harmful. It is what you do with it that could make it harmful. Just because you do not like the information or the views of others, not matter how extreme, is really not a reason to start burning books.
There is also the question of what to block - what if this harmful content is expressing a religious view? Are we happy to ban religious or even political views being expressed?
Standing up to bullies
There are a lot of problems with all of this, but how exactly could the government force filtering on the major ISPs? It could be done using legislation or by bullying the larger players with the threat of legislation. I was always told to stand up to bullies.
If ISPs are bullied in to providing filtering themselves then we end up with a system that is totally uncontrolled. There is no recourse when a site is incorrectly blocked. There is no review, or oversight. This is bad.
The only good thing about not using legislation is that AAISP can continue to offer unfiltered Internet access to those that want it.
Changing the rules
For this to work at all, it has to be acceptable for someone to intercept the communications between two other parties without their consent. That is meant to be illegal. You also have to throw away the ideas of net neutrality if you allow ISPs to treat different information on The Internet differently, blocking some and allowing others, especially if this is not done by legislation.
The other problem is that blocking has side effects. It is one more complexity and one more thing to go wrong within an ISP network - there was a case where wikipedia access was seriously affected because of child abuse image blocking. But a far more serious issue is the mis-classification of web sites. This is something we already see with mobile operators that block "adult content" by default. Web site owners do not even know they are blocked, and when they work out what is happening they have no way to address the issue, get correctly classified, and get the block removed.
Blocking is ineffective
The other problem is that blocking is ineffective. This can easily be seen by the court ordered blocks on The Pirate Bay. There are web sites dedicated to listing the hundreds and hundreds of alternative names, proxies, and mirrors that exist to allow access to The Pirate Bay bypassing the blocks.
Blocking harmful content will not work - if someone wants content to be seen then it will be seen, and adding the tag of "what the government does not want you to know" just makes it more appealing.
Thin end of the wedge
Getting all ISPs to block any content, even content everyone agrees is bad, is the thin end of the wedge. It is easy to see how the blocks can extend to extreme political views, or just wrong thinking and how any government can get to that by small justifiable steps. But even without this level of paranoia, there is the risk that corporations can ask courts to add blocks as seen by The Pirate Bay orders.
This biggest issue here is that we are considering another step to kill free speech. Free speech is a cornerstone of a free and democratic society, and the first to go in oppressive regimes. If we are considering state mandated censorship of The Internet then we are taking a big step and letting the terrorists win.
Have terrorists won already?
I wonder sometimes if they have already won. I was shocked to realise recently that when I go in to London, and take my big camera with me, I am not at all worried about being a victim of a terrorist attack, no, I am worried that I will have to argue with some policeman about my right to take pictures in a public place. Who is causing concern in that case? Who is causing terror? Who is the terrorist?
Doing the right thing
We should stand up for the freedoms we have fought hard to win and not change our ways at the whim of terrorists. Free speech means we see the good and the bad - we see the extremists but we also see the majority with the sane view and sensible counter arguments. I think we grown up enough to tell the difference without a nanny state watching our backs.