You must declare to DVLA. As long as they issue you with a licence (usually 3, 2 or 1 years), then you have an approved licence. That's what your insurers need to know. Make sure you know the guidance about when to test and what levels are needed when driving.
Cars through Motability are only available to people who have Higher Rate Mobility of Disability Living Allowance or Enhanced Rate Mobility of Personal Independence Payment, which is very unlikely for someone with type 1 diabetes, unless they also have other health problems or complications.
Personally, I drive my own car, when appropriate, but cycling 5 miles each way to work and cycling 2 miles each way to orienteering tonight is more convenient, cheaper, better for my health and the environment, plus means I can have a beer in pub afterwards.