Then I think the answer may be No, as the child/teen is too young to understand the balance of (substantial) harms versus (negligible/uncertain) benefits.
One of the legal commentators I read picked up on this. The judgment actually says:
Ms Morris submitted it is not for this court to determine clinical disagreements between experts about the efficacy of a treatment. We agree. That is a matter for the relevant NHS and regulatory bodies to oversee and to decide. However the degree to which the treatment is experimental and has, as yet, an unknown impact, does go to the critical issue of whether a young person can have sufficient understanding of the risks and benefits to be able lawfully to consent to that treatment.
It's all about the lack of evidence of efficacy in the judgment. How can the child understand in that situation? It's not clear how they would deal with evidence of lack of efficacy.
In a potential (and I think likely) future situation "there are several of studies showing puberty blockers don't work, but there are private doctors willing to prescribe them, and children caught up with groups saying they need them", then what?
And also, what about the parental consent thing? The court totally skipped that issue, because the Tavistock said they wouldn't do it, but then that Leeds hospital(?) and another popped up saying, "well we'd like to".
I wonder if maybe the Supreme Court isn't happy with some of the fuzziness here.