Your main diabetes-related requirement at university accommodation is probably a fridge in your own room, something that not all students are allowed. So, they probably just want confirmation that you use insulin. Then, when your accommodation is confirmed, you can register with a GP near to the halls / flats etc. Some universities have accommodation spread over quite a wide area, so the same GP practice won't necessarily cover all places. I suspect it's a case of the academic side of the university not passing on the evidence you have already supplied to the accommodation part. Where are you going?
Hope you get the results you need and your course and time at university all goes well - remember it's not just about studying.... During my first degree I managed to do quite a bit of cross country skiing & fell walking (must admit I chose the city for outdoor opportunities as well as academic quality), learned to SCUBA dive, drank a fair bit of beer etc; during my second, saw a few films at club, heard Tony Benn speak, spent 5 months researching in Northern Ireland, drank more beer, plus I worked part time during both degrees.