Well, you kind of answered your own question. Much of your actual vision occurs in the brain. Your eyes are like little photochemical cameras and the optic nerve is the interface cable to the vision centers of the brain. Many health issues, not related to the vision centers or the eyes and optic nerve can cause the sparkles you describe. Some can be very serious, but many are actually fairly benign conditions and easily managed. What you are seeing is similar to what people referred to when they say, "If you keep that up, I'm going to make you see stars." Those are the same sparkles seen by someone who is semi-conscious on the verge of passing out, or who has been "knocked upside the head." People see them when they strain on the toilet due to extra blood pressure from pushing so hard. Women giving birth often see them. People hyperventilating see them. People who have been dieting too heavily...stoop over to pick something up...then see them when their blood pressure dips as they stand upright again. People see them when they are experiencing hypoglycemic episodes from not eating on a schedule. People see them when they are diabetic and haven't eaten properly. Ocular Migraine sufferers see them and other visual disturbances...yet, often experience no pain at all as with a regular migraine.
So you see, a lot of things can cause your visual oddness, that have absolutely nothing to do with the eyes, or even the visual centers themselves. Your GP has surly run blood work to be sure you don't have blood sugar, thyroid, and hormonal issues that may cause those problems. If he/she hasn't, perhaps you should ask for a referral to a hormone/metabolic specialist or to a neurologist. An optician cannot run an MRI of your optic nerve to be sure there isn't a tumor on it, you would need to see a medical doctor specializing in the eyes, an Ophthalmologist....but if there is a tumor involved, the Ophthalmologist will see other signs of problems when doing his/her physical examination of your eyes and motor skills. Since you've been for vision checks, and there have been no signs of those problems (an Optometrist would also have noticed something wrong and referred you to an Ophthalmologist). If your GP has run blood work including a thyroid test, checked out your heart for blood pressure for high/low pressure issues, and cannot determine the cause, you should ask for a referral to a specialist. (Hypoglycemia for example, doesn't show up on regular blood tests. Nor do "dips" in blood pressure from some conditions.) I'm guessing it will turn out to be metabolic/hormonal, or neurological.....hopefully it will be something as annoying..but benign as ocular migraine.