Welcome to the forum
@RubyP, I do understand your concerns, as I am terrified at the moment, but as diabetics we are not the only people who are worried, there are lots of other people with underlying health issues who are not on the list to receive the letter, and lots of people on this forum who have multiple heatlth issues who did not get the letter, I myself have type2 diabetes, asthma, impaired kidney function, and liver issues, my 85 year old father who has heart problems didn't get a letter, for me part of the issue was the way they the government initially described lots of us thought we would be on the list, but were not.
I feel pretty sure that the Asthma UK, The British Heart Foundation, Liver charities, Kidney Chartities, Cancer charities, Dementia Charties and all the many many charities and organisations that offer assistance to people with particular health problems are getting the same concerns raised with them.
I know this doesn't actually help you, but you are not alone in your fears, but simply there are too many people in this country with underlying health issues for us all to get these letters.
Take care and just do the best you can in these difficult times. I have not been out of the house in a month, but I do have a key worker husband, so as he has to go out anyway he does our shopping.