As ummm correctly says, the term “next of kin” has no legal meaning or definition. You can call anybody your next of kin and it makes no difference because the title has no standing. The information in the “free dictionary” link provided by danyk says this:
“The blood relatives entitled by law to inherit the property of a person who dies without leaving a valid will, although the term is sometimes interpreted to include a relationship existing by reason of marriage.”
This is not correct. If you left your estate to “my next of kin” it is about as useful as leaving it to “the old man who lives at number 42”. It is meaningless. It is nothing to do with dying intestate and the intestacy rules lay out a clear “pecking order” of who inherits without mentioning the term. Pension schemes and Life Assurance companies may ask you to nominate a beneficiary but that has nothing to do with “next of kin”. As OG says, it would be more helpful if julu told us why she wants to know the answer to her question.