The Digital Economy Act itself (the Age Verification bits are in part 3)
Explanatory notes to that act.  Included at the end of these are links to the various debates and committee stages in Parliament.

Further government documents about the act which include:

Guidance on commercial providers (currently in draft format)

Draft impact assessment

Guidance to the Age Verification Regulator (the BBFC)


Other legislation

Criminal Justice and Immigration Act 2008 (the legislation on extreme pornography)

CPS charging guidance relating to Extreme Pornography offences

Legislation is linked to latest version on legislation.gov.uk which may not be up to date with latest amendments.