Hi bedshaped,
I bet if you ate the same meals at the same time every day there would still be differences in your readings. I have tried this and know it to be true for me. Use all the food groups at every meal.
Your body may have more stress on some days, you may have exercised more or less, you may be sickening for something , if female, it may be the time of the month, etc.......
You mention eating toast on it's own, carbs on their own will certainly raise your blood sugars.
How about making your own Granola, Muesli and then you know which ingredients to put in? Plenty of recipes online and you can adapt your own.
Hope this helps,
Catherine.