SF Site - Confidently calls itself "The Homepage for Science Fiction & Fantasy". It has news, reviews, forums and lists. Despite its old-fashioned look, it's one of the best sites around!
SF Book - This website is well designed to showcase the very best fiction, most of which will be within the genres of Fantasy, Scifi or Horror. The website is one of the oldest of it's type still going, beginning way back in 1999.
SFX Book Reviews - The book review section of the sprawling British SFX site. The reviews cover both fiction and non-fiction. SFX is a site you can easily get lost in...
RT Book Reviews - Simple, colourful and easy to use site - they claim to have over 40,000 reviews on the site!
The Guardian Newspaper Sci-Fi Reviews and News - News, reviews and well written articles from the British newspaper. It tends to write about the more literary end of Sci-fi and reviews some of the less well known UK titles.