According to Eleanor, one of the nicest readers anywhere, very few of the books I have found for free are listed as free in the UK (amazon.co.uk). There are many that are free in the Amazon UK store. If you go to the Kindle book section, you will see the Top 100 Paid in a tab on the right side of the web page (you may need to scroll down a bit). Look again, and directly to the right of that is another tab labled Top 100 FREE. That is a great place to start.
Amazon makes it so easy. You know we're all going to go blind reading all these good books right? Do try to get outside every now and then.
OH... this works in the Amazon US store too.