The work of the ASA

Although OfCom now regulates most of our media (the Tories may change this), there remain key exceptions, such as the PCC (Press Complaints Commission), and the ASA.
The Advertising Standards Authority are charged with ensuring that ads published in the UK (the Web makes this role slightly fuzzy) do not contain inaccuracies or misleading statements, and also meet a range of existing laws (eg alcohol should not be seen as enhancing sexual attractiveness; tobacco advertising is banned; restrictions on advertising confectionary to kids, etc).

You can find a wide range of examples of their work reported at The Guardian, eg:

• An Israeli tourism ad banned for depicting Palestinian territory as part of Israel;
• a family planning ad broadcast when u-16s were listening;
• a car ad banned for misleading claims on its green credentials;
• a Home Office ad on policing banned for misleading claims.

See more at http://www.guardian.co.uk/media/asa

The ASA's own website also has a wealth of useful info: http://www.asa.org.uk/
"The Advertising Standards Authority (ASA) is the UK's independent regulator of advertising across all media, including TV, internet, sales promotions and direct marketing. Our role is to ensure ads are legal, decent, honest and truthful by applying the Advertising Codes."