Was your Nikkah done outside the uk in a country it was legally binding?
I think the issue is you’ve married a very very devout and strict Muslim, who will not wish to be seen to have married a wife who doesn’t obey him and practice in his beliefs, he would prefer to threaten and end it rather than face that shame in his eyes in his community , he will also fight you to take the children.
If the nikkah was done in the uk, or a country it was not legally binding as that means you’ve no protection and he knows this, as you’re not legally married just conhabiting in the eyes of the law, so you’re entitled to nothing.
i think you’ve got yourself in a very difficult situation if im honest. You’re likely cohabiting with this man, no financial protection, and had children with him, and he’s a very strict devout Muslim, with set beliefs.
however you can just walk, you’re not married, it’s the upside, and it’s unlikely he can care for the children in amongst 5 times a day at prayer, his commitment to his religion, potential work, you will get potentially get benefits, and you can take the kids with you,
there is no easy answer here op. You either continue to live like this, or make the break and focus on keeping your kids.