I haven't read all your story so I don't know what you have done in terms of getting legal advice. Are you sure that you could go back to the UK & file for divorce there?
I would say protect yourself legally and financially first and foremost. Do not use legal action or the threat thereof as an emotional bargaining chip. Telling the other person what legal action you intend to take is often a grave mistake. Take the action, first.
I am reading a good book now called "The Powerful Self" by Steven Stosny,, the same guy who does the abusive relationship work. It has a lot of exercises and it is really helping me move out of those feelings of powerlessness. You might want to check it out.
