As your a Type 2 diabetic, and provided that you comply with the UK legal regulation in being able to read a vehicle number plate at the required distance, it shouldn't be necessary for you to inform the DVLA in this instant.
However you do need to keep an 'Eye' on this situation to ensure that things don't deteriorate and if you are worried then get a qualified Optometrist or Optician to examine you and confirm that in his/her opinion you are safe to drive.
As with any form of diabetes, it is so important in the long term to achieve good stable blood/glucose control so that these sort of problems are prevented in the first instant or at best controlled or reversed.