Most employers will have some sort of induction process where lots of "stuff" is discussed, and information delivered. That'll probably cover things like loos, lunch breaks, who makes the coffee, and when, absence policy and what anyone unwell needs to do to let the business know they won't be attending, and so on. At some point, I'm sure they might ask if you have any questions, or anything you want to say. That might be your opportunity.
As a T2, who is controlling by diet and exercise, I wouldn't feel the need to disclose anything to an employer (unless asked directly. I wouldn't lie). However, as a T1, who will need to test and inject during the day, I would be inclined to let someone know. Leaving aside a hypo/hyper situation, it could be alarming for an unsuspecting colleague to notice you injecting. They might add 2 + 2 and make a whole drug abuse conclusion.
I wouldn't make a big announcement, just mention you are diabetic, and have been for a long time, and stress (provided it's true) that it's just a part of your daily life. They may then have some questions of their own about what they might need to know if things go askew for you. People are, generally, nice and I'm sure they will be looking forward to you starting there, just as you are.
Good luck in your new job. It's an exciting time, and a big step in terms of building yourself a successful and independent life.