There isn't a definitive answer to that question I don't believe. It would depend on the location and religion of the child and the circumstances surrounding the need for fostering and adoption. Adoption became legal in 1928, by saying that it does not mean that it was illegal before then, just that there was no legislation that seemed to be in place to monitor the process.
Fostering seemed to be informal, overseers of the poor had farmed out poor law orphans and abandoned children in the early 19th century as servants, often indentured too, but into the 20th century, with the Ministry of health being established and childrens homes like Bernardo's growing across the country, legislation was needed.
Saying that, informal fostering probably continued in the nuclear family as would have informal adoption of illegitimately born children in a family.