You still need to meet the minimum spec required to get an interview (otherwise anyone with a disability could throw in a form and expect to be interviewed to be a pilot or brain surgeon without relevant qualifications or experience). Interviewers are supposed to be a bit more flexible about the spec though, so it may not hurt. You'd still need to be able to do the job, so I don't think it is wrong to put it down if you want to.
Personally I tend to put it down if not doing so would be lying, i.e. if the form asks: do you have any long term conditions such as heart problems, asthma or diabetes, not ticking yes to that would be consciously not telling the truth. Whilst you can't be sacked for having diabetes, you can be for lying on your application form.
(For T1s) Whether you consider yourself disabled or not (and using the social model of disability, it's up to you) the law would consider you so with regards to the Disability Discrimination Act - you can't choose to ignore your rights under legislation (although obviously you could refuse to follow up a discrimination case etc.).
Personally speaking, given the fact that the coalition government is looking to make it easier to fire people and harder / more costly for people to bring tribunal cases, I'd try to keep every bit of protection going, i.e. join a union if possible, make sure you have legal expenses cover on your house insurance and let HR know you are T1.