I'm not sure about the UK... but here in Australia there is disabillity discrimination law which basically says you're disabled if any part of your body doesn't work properly... so that would class diabetics as having a disability. UK is probably similar?
Ours says:
disability" , in relation to a person, means:
(a) total or partial loss of the person's bodily or mental functions; or
(b) total or partial loss of a part of the body; or
(c) the presence in the body of organisms causing disease or illness; or
(d) the presence in the body of organisms capable of causing disease or illness; or
(e) the malfunction, malformation or disfigurement of a part of the person's body; or
(f) a disorder or malfunction that results in the person learning differently from a person without the disorder or malfunction; or
(g) a disorder, illness or disease that affects a person's thought processes, perception of reality, emotions or judgment or that results in disturbed behaviour;
and includes a disability that:
(h) presently exists; or
(i) previously existed but no longer exists; or
(j) may exist in the future (including because of a genetic predisposition to that disability); or
(k) is imputed to a person.