I have give you the accurate answer. There exists no other answer that exists. I don't think you have full knowledge of UK law, and I am 100 percent certain that you haven't got the slightest idea as to how two different sets of laws about marriage (pakistan and UK) interact. Yes, muslims can have more than one marriage in Pakistan, but you do not need a visa for Pakistan do you?
You need a visa for UK, and you must follow UK law for the marriage to be valid.
As far as the passports for your kids in UK is concerned, then who told you that you must be married to their mother? Well, the law doesn't say that, and I haven't mentioned it either. You yourself are cooking stories in your head and asking questions based on village stories.
regards