Can an employer sack me for having diabetes?
If your diabetes renders you incapable of performing your job, despite reasonable adjustments and the possibility of moving to a different role being considered, you could be dismissed on fair grounds.
If your diabetes has lead you to be off work for a long time and you are unlikely to return to work soon, an employer could be acting within their rights if they choose to terminate your position. What is seen as a long time will depend on the type of work you do.