chances are you will not be able to see your gp on that day anyway, you may be waiting days for an appointment. for this reason alone an optometrist should be your 1st port of call. after which he can judge how quickly you need to be seen and by which specialist- neurologist, ophthalmologist....
A "general " practitioner is just that, some one who knows little bits about alot of things, they cant know everything.its also cheaper for the NHS for a px to be seen by their optician, rather than the patient going upto A+E especially if there was no need