Ellroy without giving away any identifying items of your own story I wonder if it would help to start your own thread and ask for advice from anyone who has been in the S.G.O. situation?
I honestly know nothing about this so can't advise at all but looking on this site....
www.education.gov.uk/childrenandyoungpeople/families/special
It says "Special Guardianship was introduced as a new permanence option for children in December 2005. It provides a legally secure foundation for building a permanent relationship between the child and their special guardian, while preserving the legal link between the child and their birth family."
So your ideas about a memory box sound very good in relation to preserving the link.
I also think this site may be helpful..
www.afteradoption.org.uk/are-you-adopted-adult-or-birth-relative/birth-parents-adopted-child
This may not be relevant for you ad please do not be offended by any of the language... I understand it is an SGO and not adoption but I am sure the feelings and some issues may be similar.
It says...
"Most parents who have given up a child for adoption say that it is the most difficult decision they ever had to make in their lives. In the past the needs of birth parents were rarely considered and it was felt that, in certain circumstances, adoption was the best option for your child. At After Adoption we understand that, although you may have lost parental responsibility for your child, you will never lose the feelings you have for your child and these can affect your everyday life."
They help with feelings and emotions, counselling and support groups, contact etc.
I know that this may not all apply to you and some may not be relevant but as a place to start this may be a place. They have a phone number and email address. Good luck.