You can get an e-version of The Writers and Artists Yearbook for twenty something quid. It's very handy. Also, as others have said, find some books yours would sit by in a bookshop and approach their agents, mentioning their work on those authors. I was fortunate (or so I thought) to get signed by a major agency right out of my degree, but that turned into a bit of a nightmare, so once I've finished this project, I'll be on the hunt for a new agent! From speaking to other writer friends, I'd say prepare yourself for a lot of disappointment and rejection, but keep heart! If your work is ready, the right agent will be out there. I'd start by approaching a small handful. 6-10. See how they go. If you're getting no nibbles, maybe tweak your pitch or your opening. You'll need a huge slice of luck to go with your talent, so be resilient and believe in yourself! Often rejections are nothing to do with the quality of your work. Good luck. If you've started, how are you getting on?

@Gabby Martini I think I was too easily swayed away from all my instincts about the project and put too much stock in the authority/knowledge of the agent. By the time I got to the end of the second draft of the novel she'd asked me to write (eighteen month's work), we could both see that it wasn't working. Instead of supporting me, she just dropped me on the spot and then ghosted me. Essentially, the lesson I learned was that any project you undertake must be for yourself. Be open to feedback, but don't do anything which compromises your vision to a point where you don't fully believe in what you're doing, because even if that thing ends up getting published, you might not be proud of it. Ultimately, an agent/writer relationship should be based on a shared vision, so don't jump at the first one that shows interest if you're not fully convinced by what they're saying!

Good luck all!