Allow adult adoption in the UK

I went into care as a teenager due to abuse, and since ageing out of the system I grew close to my former foster sister. She suggested that her and her husband could adopt me to make me officially part of the family, and so that I could officially cut ties with my biological family, but we were devastated to find out this isn't possible as I'm over 18. I know there are many circumstances where adult adoption may be wanted, and due to my past it is very important to me that it should be possible.