Just to clarify, although it seems to be getting clarified anyway, a psychologist DOES NOT have prescribing rights. We (yes, I'm afraid so) do not have medical training so we are not able to prescribe medication. However, we are able to diagnose conditions in certain situations.
However, I would suspect that, in cases of GID, a psychologist would only be able to go so far as to say that the person shows signs of possible gender identity issues, or other such claptrap that lets the individual know that we can see full well what is going on but can't state it categorically for fear of stepping on the toes of the medics. A formal diagnosis would then probably need to be made by a psychiatrist.
I'm off to my GP this afternoon to talk about such things. I will feed back anything interesting that I find.