I was wondering are bran flakes with sultanas and a glass of orange juice a healthy breakfast?

Missing the protein.: That's a good start - fruit, vitamins and complex carbohydrates. What you're missing is a good source of complete protein, like low fat yogurt, egg whites or even low fat milk. Protein would make this a complete meal. There's protein I. Some sources like bran, but it's not a complete protein so it wouldn't get absorbed efficiently.