I find it incredible in this day and age that people with your type of diabetes are treated this way. It's totally unacceptable to me that a person has to resort to not taking life needing medication in order to make it easier for themselves at work, employers should be more accommodating. Thankfully most are but where they are not then we should stand firm and help them see the error of their ways. I think it's unreasonable to expect you to provide them with a note from your G.P, can't they see what you are doing when taking insulin or testing your blood sugar? Do they think you stick needles in your stomach for the fun of it?
Diabetes has been around for a long time and anybody and everybody should know that if you are T1 then you need to take insulin, certainly most professional managers anyway and they ought be more understanding. Still in the interest of playing the game and re-educating these morons then i'd try and get them some form of written evidence from the G.P. if that's what they've asked for. Also do have a trade union and are you a member? You could ask them for help in this area and in meeting with the company HR dept to try and sort this out. But for goodness sake start taking you medication as you should, because if not then in the long run buddy you are likely to make matters worse and possibly resulting in even more sickness time, and then my friend they really will be on your case.
I think it would have been prudent to have had some sort of discussion prior to this with your employer and come to some sort of arrangement on how best to manage your condition at work without it affecting your ability to do the job instead of waiting until you fell foul of the company sickness policy, unless ofcourse they do not have anything in place for staff with such conditions? Or they have ignored your plight which is another matter altogether that needs looking at. Good luck and I hope you get it sorted quickly