You will almost certainly have to define what you mean by BAME and this is likely lead to the identification of different sub-groups within that overall category. It may be that diabetes presents differently within each sub-group and you will probably have to set this out first but then you could focus on one particular ethnic group - maybe there one that has greater resonance for you, or is just more useful from a diabetes point of view. In terms of the nursing angle, the themes of patient education and medication adherence sound perfectly fine. Again, you should set out why you have decided to focus on those particular themes in the context of diabetes. I would strongly recommend doing some digging into what research has already been done on this - this will help you to shape your own research.