The only one that springs to mind off the top of my head (that is fairly visible in the UK) is Wizards of the Coast, but their output consists of tie-ins to their games.
Most major publishers have a Fantasy/SF division or imprint though.
See if you can borrow a copy of Writer's & Artist's Year Book from the library. This is a very sought after book so you may not be able to take it away from the library, so go armed with a notebook to jot down likely looking publishers. You may be able to get a cheapish copy from amazon or ebay, seeing as we are coming to the end of 2005, but publishing houses/editors changes so quickly, it's wise to get up to date information. Good luck!