[I've recruited and managed quite a few people in large, public sector organisations, so my view is given from that background.]
I started a new job in January and started insulin therapy between being offered the job and actually starting. I told the HR person during my induction, who turned out to be T1 himself! I was 'lucky' in that my workplace has no prior medical screening.
IMO I would suggest that you should let your workplace HR dept know (if it is large enough to have one) and see what they say. If the medical form asks questions where you think you really should put T1, I would do so. You are protected by the Disability Discrimination Act so they cannot rescind the offer of a job just because you are T1 (unless there is a clear exemption, e.g. you are going to be driving an emergency services vehicle etc.) You could always ask to speak to the occupational health people about it before filling in the form.
What I would say is that they cannot legally dismiss you for being T1, but in theory if they feel you were 'lying' on your medical assessment, it could cause some difficulties. The health and safety responsibilties for anyone at work go both ways - they owe you a duty of care, which would include factors around your diabetes but also you would need to tell them as potentially having a hypo at work could put you and other people in danger.
Personally, I don't think it's relevant to mention something like diabetes during an interview, which is about testing your suitability for the role unless as I've said it directly impacts on your ability to perform the duties so I wouldn't worry about that myself.