Hi BritishDad,
Welcome and hello
US and UK laws are both based on common law, so while there are differences, a bunch of stuff is the same. And even in the US, things are different from state to state, so we usually encourage people to find out how things work where they live anyway.
We can help you think about big picture goals, strategies -- the tactics might be different and whatnot because of differences that play out locally. But even more important than all that is learning how to clear the FOG (fear obligation and guilt) that many of us bring with us to these situations. It's easy to self-sabotage when you have been conditioned to believe you are a bad person.
How far along are you with custody? Have you talked to a lawyer/solicitor yet?