Contact the carrier you are booked with. It is true that they may not believe you and insist that their options are OK. In that case - a letter from your doctor.
Contact all the airports that you are passing through and ask them what is permissible. The option of buying from airport eatieries can be one of Russian Roulette for the diabetic. I know enough people who are to understand.
Rather than tins, perhaps you can take Tupperware (or similar)? That allows the container to be opened. I am thinking that some mothers who have taken expressed milk with them had to drink some in front of security to prove that it was harmless. Not fun - but that's how it is. If you can open and taste the food, then that is something. I suggest that you DO NOT offer it to them to taste themselves, as that could be construed as aggressive and they may think that you are trying to incapacitate them with something in the container.
Above all - do not turn up unannounced!!!! Contact them all in advance. Good luck!