There are the court ordered blocks of some torrent sites. Any ISP that receives valid court documents has to implement these blocks, but they're easy to bypass.
There are the Internet Watch Foundation cleanfeed blocks which only block images of child sexual abuse and sites with those images; or images of criminally obscene material (which is a tiny subset of porn). Most UK ISPs comply with this with I think only A&A not.
Then there are the optional usually opt-in filters that cover whatever the ISP / customer wants - these are usually granular with a bunch of options for different content filtering. Mobile phones have had this for years, and it's been opt-out on mobile since introduction.
It's important to know the difference so that complaints againstthe blocking are more effective.
http://www.dailydot.com/business/david-cameron-porn-firewall...
How are they shaming people who choose to remove content filters? It's entirely private is it not.
So less a draconian measure and more a market differentiator perhaps? Plenty of families would voluntarily sign on to this feature for the peace-of-mind.
(With the exception of the 'Cleanfeed' filtering which is supposed only to prevent access to verified child pornography and seventies rock album covers)