Now, as always, I'm totally for the protection of children and others who don't want to see this type of material but it concerns me when the government talk about forcing things, it normally results in a 'one cap fits all' type scenario which seldom works.
Instead of attempting to force something like this, the government would be better placed attempting to educate parents and guardians of the various methods ALREADY available, two of which spring to mind instantly and the most obvious being OpenDNS, and freely available programs like K9 Web Protection.
Configuring OpenDNS involves changing the DNS settings of the home router which isn't rocket science and most people that can browse the web comfortably could find instructions on how to do this or could even phone their ISP helpline to be talked through the process.
Installing and configuring something like K9 Web Protection is no harder, although it could present some challenges when kids have passwords on their computers and refuse to allow parents access. I remember, not so long ago, my 15 year old daughter used some choice words on Facebook, so I embarrassed her publicly and her response was to remove me from her facebook!. I then duly disconnected her from the family router until she agreed to re-add me to facebook and needless to say it took all over 10 minutes for her to cave in and start toeing the line online !!.
This is parental responsibility and duty, yet most of the friends and indeed family I know of have no form of filtering in place whatsoever - I have OpenDNS and used to have K9 Web Protection for my daughter but this was mostly to shut her computer down at a certain time of night but it only took a few months of that before she realised I wasn't messing around any more so now she understands I mean business

As I usually say, it's the parents responsibility to ensure the family is safe online, and rather than trying to force filtering at ISP level, both the government and ISP's would do better to educate people on how to implement existing solutions, such as OpenDNS which would be a far deal more cost effective, a great deal quicker and most probably far more successful !!