Here is a non-lawyer's summary of the law relating to adoption :-
The "threshold" isn't actually the reason why a child is adopted, it is just some event or events which (allegedly) justify the State interfering in the private life of the family, to protect and promote the welfare of the child.
If a care order has been issued, a placement order, for the placement of the child with prospective adopters, can be more or less rubber-stamped on the basis of the care order, and an adoption order can be more or less rubber-stamped on the basis of the placement order.
So the key step is the issuing of the care order. The conditions for doing so are in section 31(2) of the Childrten Act 1989, as in the web link below. The most relevant words being :- A court may only make a care order .... if it is satisfied ... that the child concerned is suffering, or is likely to suffer, significant harm, and ...that the harm, or likelihood of harm, is attributable to ... the care given to the child, or likely to be given to him if the order were not made, not being what it would be reasonable to expect a parent to give to him
A key point to note about the above being that it doesn't say "is suffering and is likely to suffer", it says "is suffering or is likely to suffer".
It is sufficient that the child "is (allegedly) likely to suffer significant harm" in the future, if left in the care of the parent(s).
Essentially all that is required to get a child forcibly adopted here in England is a report from eg an "in private practice (in the pocket of the LA) psychologist" along the lines that the child is likely to suffer significant (whatever that means) harm in the future, if left in the care of the parent(s). The psychologist may typically interview the child and or the parent(s) for a couple of hours each.
Section 31(2) of the Children Act 1989 is here :- www.legislation.gov.uk/ukpga/1989/41/section/31
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I think my guesstimate, of 50% of forced adoption cases being based on predicted future significant harm, as opposed to actual current or past significant harm, is quite reasonable.