I am in the same position. Diagnosed T2 recently (HbA1c 49). Not on medication, aiming to control by diet and exercise.
I have holiday booked for January. I have Annual worldwide travel insurance taken out last February.
I haven't contacted the Insurance Company yet, I'm waiting until my next blood test results in November.
You do have to tell the Insurance Company. Failure to notify any existing conditions will invalidate your insurance, and should you need treatment you will be liable for any medical or repatriation costs yourself, which could amount to tens of thousands of pounds.
I don't know what the Insurance Company will do. They might exclude any diabetes related conditions from cover, or they might want an increased premium. Perhaps others who have been in this situation could tell us.