Some of this has already been mentioned by others here, but reposting in a reverse time line will make it easier to follow. As well as the name Forrest, I don't think "Elsie" was an Elsie.
On the Oct 1922 passenger list, Elsie Koselj gives her father as H. Weigand, of 85 Southampton St., Camberwell.
Using an address search for the 1921 census, which can't be discussed (copyright reasons), has her father mistranscribed as "Henry WeiYand", and matches perfectly for the following 1911 census:
1911: Henry Weigand ,is a baker, and married to Lina (Carolina), and he has indicated the first 3 children are from his 1st marriage, and mistakenly says these children are born in Germany, as he was. They are living at 155 Great College St Camden Town London N W, St Pancras.
Jumping back to 1901, Piece 148 Folio 41 Page 18, Henry the baker is with his first wife, Louisa, with children Louisa and May, at the same address as in 1911.
This address is also the one on his citizenship papers at Ancestry, which gives his parents names, if you are trying to work back. Note there is a George "Walter" transcribed as living with them. If you look at the image, I believe it says George Wetter.
This census shows the first two born children, Louisa and May ( transcribed as Louise and Mary) as being born in Pancras.
I believe "Elsie Louise" is actually Louisa Marie Annie. I had hoped her mother was nee Forrest, but no such luck!
WEIGAND, LOUISA MARIE ANNIE WETTER
GRO Reference: 1899 M Quarter in PANCRAS Volume 01B Page 106
WEIGAND, MAY WETTER
GRO Reference: 1900 S Quarter in PANCRAS Volume 01B Page 95
Hopping forward to 1919, Jorose posted (Reply #11) about the birth of Harold Arthur Forrest Koselj.
The details from that are DOB 30 Jul 1919, born at 41 Jeffreys Road, father Ivan Koselj, a journeyman hairdresser of 215 Euston Rd., and mother E. L. Koselj of same address.
41 Jeffreys Rd was apparently a maternity hospital, according this:
https://www.flickr.com/photos/sarflondondunc/2131043168I may have missed reading it, but is there an address for Harold's adoptive parents around that time? Maybe they were neighbours?
Edited to add: There is the possibility that Ivan wasn't the real father, and Elsie had an affair with a Forrest while Ivan was in New York? Ivan might not have even known about the baby?