Holy shit. My god, I'm just thinking of the parents.
"Sorry, we can only save one of your kids."
"You can't give Nala the treatment?"
"Well, we could. We won't, though."
I'm sure there are good reasons, but Jesus, that's harsh.
In the case of the child who already shows neurological symptoms, the treatment can help stop the progression of the disease, but most likely not reverse the already existing damage. However, there are already treatments approved to help stop the progression of the disease, including enzyme replacement therapy (provide the enzyme in charge of degrading the sustrate through the bloodstream every so often).
Some would argue that if treatments with similar outcomes already exist and are approved, it could be unethical to test this experimental treatment on a patient (I disagree).
Does that mean clincal trials didn't show a benefit for patients who had already developed symptoms? Does it mean it wasn't studied in patients who had already developed symptoms?
I don't think it's unreasonable that the NHS won't spend large amounts of other people's money on a treatment that hasn't been studied or wasn't found to be effective.
It likely wasn't studied.
It's a pretty new treatment, so the vendors start with clinical trials for use cases where the clearest/greatest impact is expected, and maybe later on move to related use cases.