So sorry for your situation, I cannot imagine how you are feeling, I think shock would be an understatement, not only are you coping with his betrayal, but also, the way your children must be feeling, you must feel so lost, really not knowing what is true, and how your life is or isn't, or what you are supposed to be (married or not).
If your his first wife, then by British law, you are legally entitled to be treated so, meaning marital status is your's, but I dont know if you had a British legal wedding , or not, so personally, being a woman, with children, not sure how old your children are, the first step for myself, would be to go seek citizen's advice, what have you got to lose? During a chat with a trained person,who may be able to give you legal, and financially, some good advice, that may at least, make you feel, that there is light at the end of a very upsetting road for you i'm sure. Best of luck in your families future plans,