Bad parenting is the real problem, and bad parents will simply allow the filter to be enabled and believe it protects their children, even though the filters are easily (even trivially) circumvented. Parents need to supervise and educate their children about internet use, not rely on filters of dubious effectiveness.
It also sets a poor precedent that objectionable content can be blocked at the ISP level in the name of protecting children, who are in fact being harmed more by poor parenting.
I'd rather not be thrown in with the like of Bahrain, Belarus, Burma, China, Cuba, Iran, North Korea, Saudi Arabia, Syria, Turkmenistan, Uzbekistan, and Vietnam for not being able to view perfectly legal content.
And don't say its for the children ... thats a parents job. You don't want them viewing porn learn to use parental control already available.
Please sign the petition, 100,000 signatures and it will be considered for debate in the House of Commons: http://epetitions.direct.gov.uk/petitions/51746