https://www.caa.co.uk/Aeromedical-Ex...e-material-GM/
Seems like Type 1 diabetes will be unfit, but Type 2 can pass subject to tests and you have to prove that you have good control of your blood sugar. However, p.12 of the certification guidance mentions pilots with insulin-dependency can still get a Class 1 provided they can demonstrate excellent control of the condition. Just my 2 cents after a quick scan. Might be best to consult an AME for the final word on the matter?