I'm almost as horrified by the blind ignorance of posters on this thread as by the literally criminal negligence of your teachers, doctors and psychologists, @Justhereandthere. You've been grossly betrayed by your own family & friends, everyone you should've been able to turn to and, of course, the male supremacist pig you were trafficked to. I'm chorusing with others that what you've achieved so far, alone, is incredible. You are, in fact, amazing!
I wish I knew the ins & outs of our ever-changing immigration policies, as it's pivotal to your course of action. I'll begin by urging you not to sign the visa. This will make him very angry, yes? So you've got a maximum of 6 weeks before the next bucket of manure hits the fan.
Divorce is an obvious next step - it's just that you have some extra complications. You can do this.
I do know parents can apply for copies of their children's passports - and that there's something you can do to prevent this. It is advisable to hide their current passports and ID documents, somewhere out of your home. Then please, please get advice from both Women's Aid and networks like the Sharan Project. It's also worth contacting Rights Of Women, a feminist legal practice.
I wouldn't judge you for leaving your children. Your posts, however, don't read as those of a mother with no inclination to mother her children. If there's the slightest chance of your spending the next few decades in agonised regret, it makes sense to figure out how to get rid of the husband while keeping your kids, your career, your savings and your home.
Don't give up. Gather up the young woman who stoically weathered years of rape and violence, who nevertheless worked out how to make her life successful, who teaches today's children to be strong & powerful - gather her up, recognise her magnificence, and guide her into the next stage of the life she always deserved.
You have the spirit. Go well, strong and wisely.