Not unnaturally, the small independent booksellers of this world (e.g. Clive Keeble) get steaming mad when the likes of me provide a link to an online bookseller and forget to mention that you can, of course, order the book in question from your local small guy and get it just as quickly.
The results are plain for all to see. But if you want the painful details, the Observer has an article by David Smith, describing the closure of several small independent firms (link from booktrade.info).
As for why such shops are closing: it seems to be mainly a question of price. The UK supermarkets are able to slice huge sums off the nominal price of a book. And, if you want to compare online prices, you can go to FetchBook.info and get a quick rundown of the various options. Currently, for instance (ignoring postage), the cheapest place to buy How and why Lisa's Dad got to be famous is a third-party trader working through Amazon.co.uk.
Please note one slight oddity about the FetchBook site. Once you have got your list of prices, you will see that the Store Rating column offers a number of reviews. So, for instance, Amazon.co.uk Market has 8 reviews. These reviews, and their associated star ratings, you will soon discover, relate only to the supplier, and not the book. More to the point, they relate only to specific traders working through, e.g. Amazon.co.uk. So Amazon.co.uk Market gets both five star reviews and half-a-star bitter complaints. And both are no doubt true, depending on who you're dealing with.