Well, at the time of writing, the answer is that, if they visit this page on the UK's Royal College of General Practitioners' website, they don't. This page says that it provides links 'to a range of eating disorder resources' and two of the most important links are broken - the links to the MARSIPAN guidelines for adults and adolescents with eating disorders.
MARSIPAN stands for Management of Really
Sick Patients with
Anorexia Nervosa and includes essential information for diagnosing people with an eating disorder. So you would imagine that it's imperative that GPs access these really important documents, wouldn't you? However the above page includes not one but two broken links to these documents.
I have emailed the Royal College of GPs to point this out.
Oh, and I've just found another broken link: the link to The Academy for Eating Disorders report 2012. Yes, 2012. Hello??? It's 2015 now???