Passports don't use that term anyway, Daisy. They simply refer to "two relatives or friends".
While 'kin' (as per the dictionary definition) only refers to relatives, it's often used as loose shorthand for 'kith and kin' (which includes friends).
But, as you indicate, 'next of kin' isn't precisely defined in law. Sometimes the phrase is used by people seeking to find out who will inherit upon intestacy. (Under such circumstances there are clearly defined rules but the legislation doesn't actually refer to 'next of kin'). In other cases, such as telling a hospital who should be contacted in an emergency, any reference to 'next of kin' is totally unconstrained by any legal considerations. (You can put down David Cameron and Victoria Beckham if you want to!).