I don't think even an optometrist is allowed to dilate. A GP may actually be allowed to do so under some circumstances, since they are medical doctors, but frankly, they know little or nothing about the infrequent (rare) eye conditions that can arise.
When I wasn't working in my optical lab or fixing network problems, I often spent time preparing educational materials for optometrists. Our ophthalmologists put on educational weekend get-aways for optometrists at least once every year to educate them on the symptoms of eye conditions that they were not trained to recognize, so that patients could be referred appropriately.
And you're absolutely right about getting the referral if he doesn't already see an ophthalmologist. Insurance companies are keen to deny payment for even what might be considered an emergency visit if they can discount the nature of the emergency with their rules. I didn't think about the OP already having an optometrist partly because I've worked with so many ophthalmologists. I won't trust my eyesight to an optometrist. My eye doctor is an old friend and I see him religiously every 2 years. Unfortunately, due to my age and incipient cataracts (too much ski-patrol work with non-UV-blocking sunglasses) he usually dilates my eyes. I hate driving home on sunny days dilated.