In its written evidence to the House of Commons Public Bill Committee on the Digital Economy Bill, the Open Rights Group put together a good summary of the problems with the approach. These include the privacy risks of creating insecure databases of the UK's porn habits, and the fact that age verification will be easy to circumvent.
As a more recent blog post by the Open Rights Group notes, MPs have finally woken up to the fact that age verification won't in fact stop children from accessing pornographic sites, and have come up with Plan B, which is even worse than Plan A: "in order to make age verification technologies 'work,' some MPs want to block completely legal content from access by every UK citizen. It would have a massive impact on the free expression of adults across the UK. The impact for sexual minorities would be particularly severe."