Here is a list of UK literary agents who represent screenwriters. Not all accept unsolicited submissions so please check their websites for submission guidelines.
It’s also well worth looking at Michelle Lipton’s blog post, a Q&A with agents Rob Kraitt of AP Watt and Matthew Bates of Sayle Screen, a great blog post from Jason Arnopp on what he did to get an agent and one from Lucy Vee Hay on getting an agent.
If you know of any others, please let me know in the comments. Good luck!