If you're working on trailer music catalogues specifically, the best thing you can do is get the quality as good as you can. Consume all the trailers you can, see how high the quality is, the dynamic range etc...find your niche. Then the next step would be get to LA for meetings if you can, or find a partner there to pitch on your behalf as that's where all the money is made.
The UK trailer/sync music market is slightly confusing due to the MCPS rate card, you can sign up with a library there too, but it's a completely different system to the US. It's more of a numbers game building smaller license fees/royalties, whereas in the US if you have a really solid catalogue that isn't very large, you can do amazing stuff and the fees are much higher, especially if you aim to keep exclusive as possible.
Build clients, develop relationships organically and never be afraid to ask for introductions from other people to potential new contacts.