i totaly agree with what miss piggy has just posted
i have sort of the same stuation with my wife[she has a son ] but the fathers name is not on the birth cert] so its not an issue and our situation is some what different
i would personaly be sorting out things like getting married and getting your wife to join you in the uk because it could take at lot longer to sort out if you want both of them to join you here asap the child can always join his mother at a later date
you will have enough problems and expence just sorting her application out