Restricting it to people with PR would restrict it to almost no-one. My understanding of PR was that both parents had to agree to it
Not since 2002.
You can apply to the courts for PR or a residency order that confers PR .. as the spouse/civil partner of one of the parents with PR.
As I understand it, having it granted depends on individual circumstances and in the case of an older child, their wishes.
I am of the opinion that with the commitment of parental relationships less clearly defined and less durable than they used to be, it would be to the benefit of children...
-in terms of knowing who would be responsible for them in a parental role, no matter what shit hit the fan
-in terms of knowing who has right of parental access/authority over them
-in terms avoiding unnecessary hot spots of tension between parents post spilt
-in terms of clear and well defined boundries when it comes to the various adults in their lives.
... if we ditched step parent as a socially sanctioned curtesy title for anybody who happens to currently attached to a parent romatically, and instead made it a clearly defined legal title in the same way say adoptive/biological/foster parent is.
I am not a fan of "faux familial" titles being employed for the sake of adult preferences. Clear cut boundries are good. I remember my mum's cousins talking of the insidious nature of being being required to call their mother's merry go round of fancy men "uncle". It made things nicer for the adults. But it led to significant issues from the kids' perspective.
I think that might be an even greater risk where PREFIX mum/dad is not clearly defined, carefully rationed and socially sanctioned only when it conforms to fairly stringent standards that pertain to legal comitments made to the actual children at the sharp end. Legal commitments that last until they reach the age majority, regardless of which adult romantic partnerships last the course, or not.