I guess I was looking for the figures, as in, how many subjects, what was the the treatment and ultimately what was the outcome.

-Its ongoing with many treatments and doubtless studies. There's extensive available material on Google if you are seriously interested: most of it US in origin.

I'm sure the Tavistock clinic will have the numbers up to present for England and Wales. On "Outcomes" doubtless in their view it will all have been a roaring success.
"Happiness" or "well being" are subjective issues and also subject to change over time. There is of course no real objective yardstick available for comparison. Someone happy to receive castration or a double mastectomy at an early age might... um perhaps somewhat regret that at a later date.

"...only six out of 64 young people eligible for fertility preservation completed the process."

-Yes that left me at a bit of a loss too. Hard to say if its the tabloid press making an arse of it or somewhat contrary bizarre jargon coming from the clinic itself: I posted that article merely to show they were dishing these out to extremely young children here.