No, there isn't. Sometimes foster parents go on to adopt their foster child after a few years feeling that they have become part of the family and want that legally recognised. There are schemes where babies are placed with potential adopters in a foster placement as it is most likely that the end result will be adoption - but the birth family has as many rights under that foster placement as any other, and the baby may be returned to the birth family rather than become available for adoption.
Once a child needs adoption the last thing they need is another failed placement by going to potential adopters who are not fully committed. Some placements do break down, sadly, but there would be even more if potential parents go into a match to 'see how it goes'.
Re: success stories. DD1, 10 when placed. Bit of a stressful teen - staying out all night rather than violence, and constant attitude towards me and charm to her teachers etc (classic attachment stuff). Now a kind, generous young woman doing teacher training, and we have a great relationship (she is the first to admit she gave me a hard time as a teen!).
DD2, aged 2 when placed - has struggled academically but now we have the right school and EHCP in place is on track to pass 5+GCSEs including English and Maths. Always very close to me emotionally, some very mild teen attitude to me at the moment (mid teens), helps out with the younger ones at her hobby, has a range of friends, comments from teachers generally along the lines of 'a pleasure to have in class'. Every prospect of earning a living, living independently, and having the 'normal' range of experiences and relationships as any other adult.
Versus friend's birth daughter, random genetic deletion at conception, has physical and learning disabilities, a regular schedule of medical/OT/physio appointments, regular trips to A&E, will never live independently. A lovely girl, and very much loved and wanted. But I have it far easier as a parent.