Before employing an agent, go to your local library! Any UK reference library will have copies of the two key books published annually for writers..."The Writers' and Artists' Yearbook" and "The Writer's Handbook". In either, as an earlier answer here suggests, you will find which publishers want to publish and which agents are prepared to handle the type of book you have to offer. These are the only ones you should approach, obviously. In addition, the ordinary lending library will have many books offering good advice on presenting your manuscript, plotting your story - if it's fiction - and so on. Just ask the staff where to find them. Definitely...make the library your very first port of call...and good luck!