I've used this site for information, directly recommended by my LA. https://childlawadvice.org.uk/information-pages/parental-responsibility/
Second, I want to say a massive thank you for the wonderful messages on the "other" thread, this community is wonderful.
I'll deal with D(S)S first. 23, I've raised him with his mum since just before he was 2. Sperm donor used contact as a weapon, D(S)S lived with him for a few weeks (under a month) in his mid-teens. Then kicked him out and told him to fuck off.
Adoption was considered around a decade ago, I love him so very much, we've always had an unbreakable, unwavering bond. Decided not to in the end, as we didn't need a piece of paper.
Another thread yesterday made me realise legally, it would be worth going for it again. So spoke to DS late last night and he's given his blessing and is rather excited. He's also freezing in -20C weather  (living abroad atm).
I've spoken to the adoption team. Their stance is a court will not award an adoption order past the day prior to the child's 19th birthday. However, she mandated that I can legally acquire PR and it's a straightforward process.
However., spam, sorry, at bottom.
So, as DP and sperm donor were never married, sperm donor doesn't have PR in the first place and we cannot remember him acquiring it. So it seems straightforward.
But. DD was born in '99. DP and I are unmarried. Therefore it appears I do not have PR for her, which has hit me like a freight train.
The salient point being: The law has changed so that unmarried fathers who registered or re-registered their name on their child’s birth certificate after 1st December 2003 will have Parental Responsibility for their child.
So any deadbeat dads out there who have fathered a child since 1/12/03 carry PR irrespective of involvement in their children's lives.
I know we need to go see a solicitor, we should be moving in the near future, so will then turn attention back to getting this resolved.
I'm ecstatic with DS response though, I think I might be having a shandy or two at the coming weekend. DS can just bury his in the snow ⛄️. Kids!
An unmarried father can obtain Parental Responsibility by:
• marrying the mother;
• having his name registered or re-registered on the birth certificate if his name is not already registered;*
• entering into a Parental Responsibility Agreement with the mother;
• obtaining a Parental Responsibility Order from the court;
• having obtained a Residence Order prior to 22/4/2014;
• being named as the resident parent under a Child Arrangements Order;
• becoming the child’s guardian on the mother’s death.
The law has changed so that unmarried fathers who registered or re-registered their name on their child’s birth certificate after 1st December 2003 will have Parental Responsibility for their child.
Therefore:
• if an unmarried father has a child after 1st December 2003 and he is registered on the birth certificate, he WILL have Parental Responsibility.
• if a child’s birth was registered before 1st December 2003 and the father was not named on the birth certificate, the birth can be re-registered to include the father’s name – the father WILL then have Parental Responsibility.*
• if a child’s birth was registered before 1st December 2003 and includes the name of the unmarried father, the father WILL NOT have Parental Responsibility (unless obtained by other means).