This is by no means a slam-dunk, and I think it will go to the Supreme Court, whatever the AC outcome.
Despite the fox-basher's claims, the original judgement reinforced Gillick. However, the judgement's ring-fencing of decisions about PBs as very unlikely to fall within an under 16's competence was novel. We don't characterise any other decision about consent in this way for under 16s (I think I am right in saying), and I can't think of any other analogy in medical law in general - with the arguable exception of life or death decisions by people with a Lasting Power of Attorney, but that's rather different, as it is not the patients themselves making the decision in that case.
So the legal principle underlying the original judgement won't be overturned by the AC, as it is Gillick. But the concept of ring-fencing PBs as particularly unlikely to be within Gillick competence is novel and may be successfully challenged by the Tavi.
Please don't shoot the messenger btw. I am not responsible for deficiencies in the legal system 