It's not the disability act any longer it is the Equality Act 2010 and it does qualify and recognise diabetes as a disability under the law. The reason diabetics don't get DLA etc is because the government do not award disability benefit to people who are disabled, they base the eligibility on how the disability effects your day to day life and diabetics, most of them, are deemed perfectly capable of living a normal day-to-day life unless, like you said, they have complications, but it doesn't change the law and it is a disability under the law. I don't know the rules around disclosing it but I know it is a disability under the law as I represented at an employment tribunal for it, it is a disability.
Diagnosed with GD in 2010, Completely disappeared postpartum. Re-diagnosed December 2012 with type 1.5 diabetes, age 26, BMI 22 currently controlled by only Metformin, 500mg twice a day.