I feel a great deal of sympathy for you zombiegirl. I put up with my GP for years before he moved on when the practice reorganised last year. In many ways I defended what he was doing on the basis that it was the system that was at fault and he was doing the best he could within it.
I was wrong. Not everything that is wrong with me is down to my weight and smoking. A new GP cleared up a mouth/throat problem, that I have suffered with for something like 12 years, with a simple cheap medication given on an almost one off basis. He looked at the back of my mouth/throat, was not certain what the problem was, but saw no reason not to give this stuff a try as it was the standard first line treatment for the most likely cause. It worked. Why have I had to suffer this long when a simple cure was readily available?
At the end of the day, it has nothing to do with the system, the 7 minute appointments or similar. My old GP simply was a poor GP because he too quickly formed his own opinions and was then blinkered against anything else. I still do not doubt his medical skill, but the way he interacted with me and I now understand his patients generally, made him a poor GP. Medical skill is not the only thing a doctor needs to be a good GP. It was his duty to work within the system to get me the correct medical care. It was his duty to tell me what he wanted or needed me to do to fit in with this, be it arranging to see him on a much more regular basis or book double appointments. It should not have been up to me to guess or find ways to work around him.
Do not let yourself get into the same situation, so that you do not get the correct care.
I would suggest that the first thing that you do is enquire of the practice if they have a doctor who includes in his/her interests diabetes care. If they have one, then suggest that in the light of your Type 1 diabetes, you should be on their list, rather than your current doctor's. Of course it depends upon the size and nature of the Practice, but this is a way of at least trying to bring about a change, without actually having to confront the issue of the current treatment that you are getting.
If he is the one with the interest in diabetes care, then the suggestion that your care be supervised by some hospital or specialist unit for Diabetes and Metabolism Care would be one way of trying to get around him. The Practice is likely to take note of anything that they say, rather than relying on your particular GP.
If this is not possible, then yes consider a formal written complaint. In the first place this should be the the Practice Manager, however if the Practice is fairly small without the more formal positions and procedures of a larger practice then you might like to consider going outside the practice. This has all been changed with the abolishing of the Primary Care Trusts (PCTs) and such complaints now go through the relevant Clinical Commissioning Group (CCG). For more details try a web search using something like 'complaining about your GP'. This should give you the up to date information and the details as to whom you should be sending any complaint to.
Final bit of advice, if you find the receptionists at all approachable, ask if they have a leaflet about making a formal complaint. You might well find that the next person to appear is someone just wanting to have a little chat and find out what the problem is and if they can resolve it for you. You might be surprised how hard some of the Practice personnel are willing to work to avoid having formal complains registered against it.