How do I get a producer/manager/agent for music composition. Specifically, soundtracks.?

I've been writing music now for soundtracks for a while now. I have
also written a full soundtrack for a play coming up in London. What I
need to know is if anyone knows how I can go about getting in contact
with an agent/manager/producer etc so they can represent me and I
could do more composition work. I have no idea where to even begin...

Any help on this would be greatly appreciated.