I think I made a comment on another topic relating to this. GPs aren't really experts, as has already been mentioned, and are the first line of defence for the NHS. That my GP was able to turn around and say, "You've got way beyond what I know, I'll refer you to the specialists." was a great result and shows him in a good light.
Some don't take it as well as that though, and feel very threatened that their whole position in society is eroded by "pseudo medical professionals with too much access to information they don't properly understand" (A statement taken form an older generation Mr. So-and-So, a Consultant friend of the parents.
Having said this, I don't believe it applies only to GP level services. Having received the advice from Professor Graham Macgregor (a very well known man in Hypertension circles, google him) that blood pressure would kill me before blood sugar control and that it was more important to manage that (I actually made a complaint to the hospital after receiving this advice), I'd say that there are plenty of disciplines in the medical world where the intelligent, questioning patient probably has as good a knowledge about what affects them as anyone else, whether they be an endocrine specialist or a hypertension specialist. Very few seem to properly take into account looking across conditions and seeing the holistic view, and the GP doesn't have enough knowledge to do that.
As a result it is left to those patients that want to to have that level of knowledge, control and power, and you have to plough on regardless (and clash heads when necessary) rather than accept the wrong advice.