There is already age verification in that you have to be 18 or over to sign up for an ISP because they are offering services on credit. The responsibility for how those services are then used by minors should remain with the adult(s) who procured the services. PAYG phones may be different but perhaps the purchase of these services should also be subject to adult approval.
The Internet is not and will never be, a safe place for children and I for one do not want my rights further eroded by more government sanctioned snooping. ISPs should not have to police our communications and any law which promotes this will probably be in conflict with the ECHR.
Then there is the slight difficulty that ISPs would face in accurately predicting what sites are actually hosting porn - particularly given that the UK's legal definition of porn is so vague and flexible. ISPs have the reasonable objection that they could never hope to fully comply with any age control legislation. After this comes the problem that porn is readily available on P2P and Usenet, neither of which would be affected by any web filter. Do none of these politicians actually bother to find out the facts before they start dreaming up new legislation?