It seems to me that you may be attempting to "rush" matters through in haste. Clearly, you will be barred from "marrying" your partner in the UK under UK law during the lifetime of your present wife in South Africa, unless, of course, you obtain a divorce granted by a Judge, or your wife dies. As to "how, where, what and when" in your post above, I cannot understand the question at all, and this needs fuller explanation from you. However, if, as you say, your wife has agreed in writing to concede responsibility for your son's upbringing to your partner ijn the UK, your partner will be known simply as a "guardian" and, as such, will assume legal responsibility for the child. It doesn't require a court order to assume that responsibility, since, as I see it, matters are being agreed amicably by your wife and yourself.