Level term assurance is possible, but have not found one that ever offers critical illness cover.....
Costs can vary enormously. Various diabetic websites and offices can help to give advice. Mine was done through comapny called Homeowners Friendly Society, as this was the cheapest at the time 10 years ago. (This may be blocked, as rules of website).
Diabetes does not prevent you from getting Life Term Insurance, but may make it more expensive. The only advice I can really give is: Buy NOW for the future, as the older you get, the more expensive it will get (diabetic or non diabetic), be honest and declare if you smoke etc.
Any ommisions can invalidate insurance should a claim be necessary......