The establishment of a national blocklist is itself a bad idea. Creating a facility whereby ISPs can be compelled to block entire websites is a bad idea on its face. The security problems raised by such a facility are grave (a hijacker could use it to block the BBC, or Parliament, or Google), and the temptation to extend this facility for use in other civil actions, (say, libel) will be great. Also, as my friend Lilian Edwards has pointed out, the LibDem proposal does not stipulate how long sites must be blocked for, nor what the procedure is for unblocking them.
4. There is no evidence that this will work. Dedicated infringers have shown a willingness and capability to use technologies such as proxies to evade firewalls. These proxies -- many of them legitimate businesses at home and abroad -- are cheap and easy to use, and make it trivial to evade ISP-level filtering. However, "good guys" (small traders, individuals wishing to share private material with friends and family) should not have to bear the expense and difficulty of evading the Great Firewall of Britain to do legitimate business on the net.
5. This is bad for the nation. The only country to enact anti-web-locker legislation to date is South Korea, which brought in a similar measure to the LibDem proposal as a condition of its Free Trade Agreement with the USA, whose IP chapter focused largely on locking down the Korean Internet