Steve, those ads are not for true self-publishing, but rather a package whereby they offer editing, proofreading, registration of ISBN and other services. They also place your book on their websites, but how many sales this generates I've no idea. Mostly too they are Print on Demand companies, so apart from printing a number of copies for the British Library for cataloguing purposes and some 'free' copies for the author there's no need to print a large first edition. I believe they advise on marketing, but basically it's up to the author. And you have to buy the books to sell, apart from the initial set up fee.
You can do all of that yourself, and either go for POD or more traditional printing methods, although if you go for litho printing you'll have 1,000 copies of your book delivered. There's lots of advice on the Net, and an excellent article by our own Richard Brown on site.
<Added>Here's the link:
http://www.writewords.org.uk/articles/self_publishing1.asp