In an ideal (istic) world Mermaids would come under a professional body as some patient charities do.
I know of one charity that's attached to the British professional body in the field for a certain condition. As such, patient experiences go back and forth to them and they give medical advice to the charity/ members. The charity effected real change in one urgent area a few years ago after many failings were reported to them by members / patients .
There are other separate charities for the condition that operate differently (though all appear to be increasingly singing from the same song sheet.)
I'm not really sure how easy that would be to run in the area of paediatric gender dysphoria, especially given the current climate though.
But it could be a stipulation of mermaid's funding and status that they follow future U.K. nhs guidelines (currently being reviewed) given the seriousness of the safeguarding implications.
As I say, ideally.
And probably overly optimistic and idealistic.