Indeed Williams and others in the Birmingham school have much to say about advertising from a Marxist perspective. Another more explicit engagement of Williams w/ Advertising is in the
Advertising: The Magic System, New Left Review I/4 July-August 1960 http://www.newleftreview.net/?page=article&view=1568
this is related to his other pieces on Cultural theory that can also be found in the NLR (If you need the article I can forward it on.)
The best versions of this are also involved in discussing the work of Barthes when he followed on Saussure's structuralist linguistics. When I have my students consider advertisments, I often start with Barthes' _Myth Today_ which can be found in a pdf here:
http://www.turksheadreview.com/library/barthes-mythtoday.pdf
Of the British CCCS work related to this, the most concise is an article I have yet to find online, Stuart Hall's "Encoding/Decoding." Here's a brief overview of the argument:
http://www.aber.ac.uk/media/Documents/S4B/sem08c.html
Though the latter is focused mostly on the construction of News discourse, it is very useful for considering the normative claims of advertising from a Marxist perspective. Though he is somewhat engaging w/ Barthes, the main theorists used are Gramsci, Volosinov and Foucault and he is mainly considering the claims of communication theorists who would use information models to think about how meaning is exchanged. Hall wants to point to the necessity of their being a dominant hegemonic discourse for a mass media communication to occur. In fact, a good place to start in thinking about the way that advertising works as an ideological tool is to consider it in the terms of the ideological sign a la Volosinov:
http://marxists.org/archive/voloshinov/1929/marxism-language.htm
Finally, if you haven't had enough, Dallas Smythe is an important critic of this kind of Western Marxism. He's an interesting fellow in general, but the critique he levels against them also provides a useful way of thinking about advertising in Marxist terms. It isn't perfect theoretically, but his concept of the audience commodity inspired quite a few people in this area, as well as a pretty vehement debate in the pages of The Canadian Journal of Political and Social Theory. Basically, the argument is that focusing on the ideological force of advertising or of the content of other media messages overlooks the relationship of the viewers with commercial forms of broadcasting, namely that, by watching TV they are working for the TV network. Their "labor" is sold to advertisers by these networks. As I said, it isn't perfect, but it does begin to talk about how much of what we learn and do is training for our being good consumers. Though a lot of Smythe's theories were forgotten, I can't help but wonder how much he was right when the president basically says that the main work of the US citizen is to be a good consumer. It messes with more traditional understandings of labor--and the latter is surely undertheorized in his work--but he seems to be onto something. If the Culturalists above focus most of their marxist readings on German Ideology and, to some extent, the Grundrisse, Smythe is beginning more from Capital. To read his article "Blindspots in Western Marxism" you can download a pdf here:
http://www.ctheory.net/library/journal.asp?journalid=3
registration is required, but it's free
Of course there is also Baudrillard's work on the _System of Objects_ recently re-published by Verso in their radical thinkers series, which takes on the notion that Use value can be assumed. I know that Doug isn't a fan of Baudrillard's later work, but I think this earlier stuff is useful. Not a whole lot of that online though so you have to head off to the library for that.
have fun.