Apple crumble and custard contains agoodly amount of fat. Fat slows the breakdown of sugars in the milk and apple.
Also, in the morning, my and many people's insulin to carb ration needs adjusting so that you take more insulin for each 15g of carbohydrate. Presumably you are having lowish fat milk (very carb full, and quite quickly absorbed) bran flakes (not that quickly absorbed) and raisins, very quickly absorbed and quite full of carbs.
Also, did anyone see the programme about dieting on the Beeb a week or two ago? they had divided a group of workers into two groups, and gave one group meat, potato and veg and gravy as a meal. The other group were given EXACTLY the same ingredients in the same quantities, only it was put in a blender and made into a soup. Then they scanned their stomachs 2 hours and 4 hours later. The group who had the food separately had digested most of their meal after 2 hours. The group who had soup still had food in their stomach 4 hours later. What happens with the meal is the water/gravy part was absorbed really quickly just leaving a few lumps of food to digest quite quickly, but he liquid in the soup meal was mixed in with the food and it all took a lot longer.
So the message might be moulinex your cereal in with your milk and raisins, and it might last longer and not raise your BG so quickly!