This is actually a really complicated situation and not a simple "AITA?" one. I think both of you are at fault here and I'll tell you why:

He's obviously being extremely dismissive of your feelings and wishes. It's selfish for him to only think of his wellbeing and to be already threatening you with a custody battle. As a child with divorced parents who suffered through one of these battles, I can tell you that once the threat is out there, you can be very sure he's not changing his mind on that. I know you said that he's kind and that the smirks and the eye rolls are the only problem, but my question is: do you need more red flags? If you can't have understanding and empathy in a relationship then you don't have much. I don't think your marriage is healthy at all.

I'm aware I sort of went on a tangent there so I'll get back to the point. As many people have already stated, you're also at fault for tying yourself more to him and to the US by having kids when you knew you wanted to go back. Making you feel bad for your decisions and attacking you isn't going to do anyone any good since you can't go back in time.

I'm not a parent, so I can't speak much on what you should do when you have to choose between your happiness and that of your children. I can tell you for sure, though, that when your kids grow up, go to college, and move out, it's going to hurt even more not having lived your life the way you wanted. Sorry if this sounds rough or selfish.

I think the first thing you should do is leave your marriage. Seriously. This guy doesn't sound good and the fact that you've been with him so long has probably blinded you a bit. As you obviously know, moving so far away is never easy for anyone, but I think that once you're out of the negative environment and you're creating a new life for yourself, being in the US won't be so bad. You can even try bringing your parents as well. I would say stay until your kids become adults then move. You even said it yourself: "he consistently disregards my wellbeing." If you can't go to the UK right now because of your kids, do this at least for yourself.